Chapter 8
MISCELLANEOUS PROCESSES
§ 74. Coating Glass with Aluminium and Soldering Aluminium.
A process of coating glass with aluminium has been lately discovered, which, if I mistake not, may be of immense service in special cases where a strongly adherent deposit is required. My attention was first attracted to the matter by an article in the Archives des Sciences physiques et naturelles de Genève, 1894, by M. Margot. It appears that clean aluminium used as a pencil will leave a mark on clean damp glass. If, instead of a pencil, a small wheel of aluminium--say as big as a halfpenny and three times as thick--is rotated on the lathe, and a piece of glass pressed against it, the aluminium will form an adherent, though not very continuous coating on the glass.
Working with a disc of the size described rotating about as fast as for brass-turning, I covered about two square inches of glass surface in about five minutes. The deposit was of very uneven thickness, but was nearly all thick enough to be sensibly opaque. By burnishing the brilliance is improved (I used an agate burnisher and oil), but a little of the aluminium is rubbed off. The fact that the burnisher does not entirely remove it is a sign of the strength of the adherence which exists between the aluminium and the glass. In making the experiment, care must be taken to have the glass quite clean--or at all events free from grease--in order to obtain the best results.
M. Margot has contributed further information to the Archives des Sciences physiques et naturelles (February 1895). He finds that adherence between aluminium and glass is promoted by dusting the glass with powders, such as rouge. There is no doubt that a considerable improvement is effected in this way; both rouge and alumina have in my hands greatly increased the facility with which the aluminium is deposited. M. Margot finds that zinc and magnesium resemble aluminium in having properties of adherence to glass, and, what is more, carry this property into their alloys with tin. Thus an alloy of zinc and tin in the proportions of about 92 per cent tin and 8 per cent zinc may be melted on absolutely clean glass, and will adhere strongly to it if well rubbed by an asbestos crayon.
A happy inspiration was to try whether these alloys would, under similar circumstances, adhere to aluminium itself, and a trial showed that this was indeed the case, provided that both the aluminium and alloy are scrupulously clean and free from oxide. In this way M. Margot has solved the problem of soldering aluminium. I have satisfied myself by trial of the perfect ease and absolute success of this method. The alloy of zinc and tin in the proportions above mentioned is formed at the lowest possible temperature by melting the constituents together. It is then poured so as to form thin sticks.
The aluminium is carefully cleaned by rubbing with a cuttle bone, or fine sand, and strong warm potash. It is then washed in water and dried with a clean cloth. The aluminium is now held over a clean flame and heated till it will melt the solder which is rubbed against it. The solder sticks at once, especially if rubbed with another bit of aluminium (an aluminium soldering bit) similarly coated. To solder two bits of aluminium together it is only necessary to tin the bits by this process and then sweat them together.
The same process applies perfectly to aluminium caused to adhere to glass by the previously mentioned process, and enables strong soldered contacts to be made to glass. In one case, while I was testing the method, the adhesion was so strong that the solder on contracting while cooling actually chipped the surface clean off the glass. In order to get over this I have endeavoured to soften the solder by mixing in a little of the fusible metal mercury amalgam; and though this prevents the glass from being so much strained, it reduces the adherence of the solder. It is a comfort to be able to solder aluminium after working for so many years by way of electroplating, or filing under solder. An alternative method of soldering aluminium will be described when the electroplating of aluminium is discussed, § 138.
Gilding Glass. In looking over some volumes of the Journal fuer praktische Chemie, I came across a method of gilding glass due to Boettger (Journ. f. prakt. Chem. 103, p. 414). After many trials I believe I am in a position to give definite instructions as to the best way of carrying out this rather troublesome operation. The films of gold obtained by the process are very thick, and the appearance of the gold exceedingly fine. The difficulty lies in the exact apportionment of the reducing solution. If too much of the reducing solution be added, the gold deposits in a fine mud, and no coating is obtained. If, on the other hand, too little of the reducing solution be added, little or no gold is deposited. The secret of success turns on exactly hitting the proper proportions.
The reducing solution consists of a mixture of aldehyde and glucose, and the difficulty I have had in following Boettger's instructions arose from his specifying "commercial aldehyde" of a certain specific gravity which it was impossible to reproduce. I did not wish to specify pure aldehyde, which is not very easily got or stored, and consequently I have had to determine a criterion as to when the proportion of reducing solution is properly adjusted.
The aldehyde is best made as required. I employed the ordinary process as described in Thorpe's Dictionary of Applied Chemistry, by distilling alcohol, water, sulphuric acid, and manganese dioxide together. The crude product is mixed with a large quantity of calcium chloride (dry--not fused), and is rectified once. The process is stopped when the specific gravity of the product reaches 0.832 at 60° F. The specific gravity of pure aldehyde is 0.79 nearly.
The following is a modification of Boettger's formula:-
Solution I
1 gram of pure gold is converted into chloride--got acid free--i.e. to the state represented by AuCl3, and dissolved in 120 cc. of water.
This solution is the equivalent of one containing 6.5 grains of trichloride to the ounce of water.
Solution II.
6 grams sodium hydrate.
100 grams water.
Solution III.
0.2 grams glucose (bought as pure).
12.6 cubic centimetres 95 per cent alcohol.
12.6 cubic centimetres water.
2.0 cubic centimetres aldehyde, sp. gr. 0.832.
To gild glass these solutions are used in the following proportions by volume:-
16 parts of No. I.
4 parts of No. II.
0.8 parts of No. III.
The glass is first cleaned well with acid and washed with water: it is then rinsed with Solution No. III. If it is desired to gild the inside of a glass vessel, Solution No. III. may be placed in the vessel first, and the walls of the vessel rinsed round carefully. Solutions I. and II. are mixed separately and then added to III.--after about two minutes the whole is well shaken up.
If it be desired to gild a mirror of glass, the glass-plate is suspended face downwards in a dish of the mixed solutions--care being taken to rinse the glass with Solution III. first.
If the mixture darkens in from 7' to 10' in diffuse daylight and at 60°F. it will gild well, and it generally pays to make a few trials in a test tube to arrive at this. If too much reducing solution is present the liquid will get dark more rapidly, and vice versa. The gilding will require several hours--as much as twelve hours may be needed.
The reaction is one of great chemical interest, being one of that class of reactions which is greatly affected by capillarity. Thus it occasionally happens that when the reducing solution is not in the right proportion, gold will be deposited at the surface of the liquid (so as to form a gilt ring on the inside of a test tube), the remainder of the gold going down as mud. The gold deposited is at first transparent to transmitted light and is deeply blue. I thought this might be due to a trace of copper or silver, but on carefully purifying the gold no change of colour was noted. If the reducing solution is present in slightly greater proportions than that given in the formula, the gold comes down with a richer colour, and has a tendency to form a mat surface and to separate from the glass. The gold which is deposited more slowly has a less rich colour but a brighter surface. The operation should be interrupted when a sufficient deposit has been obtained, because it is found that the thicker the deposit, the more lightly is it held to the glass surface.
§ 75. The Use of the Diamond-cutting Wheel.
A matter which is not very well known outside geological circles is the manipulation of the diamond-cutting wheel, and as this is often of great use in the physical laboratory, the following notes may not be out of place. I first became acquainted with the art in connection with the necessity which arose for me to make galvanometer mirrors out of fused quartz, and it was then that I discovered with surprise how difficult it is to obtain information on the point. I desire to express my indebtedness to my colleagues, Professor David and Mr. Smeeth, for the instruction they have given me. In what follows I propose to describe their practice rather than my own, which has been of a makeshift description. I will therefore select the process of cutting a slice of rock for microscopical investigation.
§ 76. Arming a Wheel.
Fig. 63.
A convenient wheel is made out of tin-plate, i.e. mild steel sheet, about one-thirtieth of an inch thick and seven inches in diameter. This wheel must be quite flat and true, as well as round; too much pains cannot be taken in securing these qualities. After the wheel is mounted, it is better to turn it quite true by means of a watch-maker's "graver" or other suitable tool. The general design of a rock-cutting machine will be clear from the illustration (Fig. 63).
The wheel being set up correctly, the next step is to arm it with diamond dust. For this purpose it is before all things necessary that real diamond dust should be obtained. The best plan is to procure a bit of "bort" which has been used in a diamond drill, and whose properties have therefore been tested to some extent. This is ground in a diamond mortar--or rather hammered in one--and passed through a sieve having at least 80 threads to the inch. The dust may be conveniently kept in oil.
To arm the wheel, a little dust and oil is taken on the finger, and laid on round the periphery of the wheel. A bit of flint or agate is then held firmly against the edge of the wheel and the latter is rotated two or three times by hand. The rotation must be quite slow--say one turn in half a minute--and the flint must be held firmly and steadily against the wheel. Some operators prefer to hammer the diamond dust into the wheel with a lump of flint, or agate, but there is a risk of deforming the wheel in the process. When a new wheel is set up, it may be necessary to repeat the above process once every half hour or so till the cutting is satisfactory, but when once a wheel is well armed it will work for a long time without further attention.
§ 77. Cutting a Section.
A wheel 7 inches in diameter may be rotated about 500 times per minute, and will give good results at that speed. The work, as will be seen from the diagram, is pressed against the edge of the wheel by a force, which in the case quoted was about the weight of eleven ounces. This was distributed along a cutting arc of three-quarters of an inch.
A convenient cutting lubricant is a solution of Castile soap in water, and this must be freely supplied; if the wheel gets dry it is almost immediately spoiled owing to the diamond dust being scraped off. In the figure the lubricant is supplied by a wick running into the reservoir. I have used both clock oil and ordinary gas-engine oil as lubricants, with equally satisfactory results. As to the speed of cutting, in the experiment quoted a bit of rather friable "gabbro," measuring three-quarters of an inch on the face by five-eighths of an inch thick, was cut clean through in six minutes, or by 3000 turns of the wheel. The travel of the edge was thus between 5000 and 6000 feet, or say 9000 feet, nearly 2 miles, per inch cut.
A good solid rock, like basalt, can be cut into slices of about 3/32 inch thick. A very loose rock is best boiled in Canada balsam, hard enough to set, before it is put against the wheel.
Instead of a grinding machine a lathe may be employed. The disc is, of course, mounted on the mandrel, and the work on the slide-rest. The latter must be disconnected from its feed screws, and a weight arranged over a pulley so as to keep the work pressed against the wheel by a constant force.
It may, perhaps, occur to the reader to inquire whether any clearance in the cut is necessary. The answer is that in all probability, and in spite of every care, the wheel will wobble enough to give clearance. If it does not, a little diamond dust rubbed into the side of the wheel, as well as the edge, will do all that is required. The edge also, after two or three armings, "burrs" a little, and thus provides a clearance naturally. It is not unlikely that in the near future the electric furnace will furnish us with a number of products capable of replacing the diamond as abrading agents. The cost of the small amount of diamond dust; required in a laboratory is so small, however, that it; is doubtful whether any appreciable economy will be, effected.
§ 78. Grinding Rock Sections, or Thin Slips of any Hard Material.
A note on this is, perhaps, worth making, for the same reasons as were given for note, § 75, which it naturally follows. Just as trout-fishing; is described by Mr. Francis as the "art of fine and far off," [Footnote: In the Badminton Library, volume on Fishing.] section grinding may be called "the art of Canada balsam cooking," as follows. A section of rock having been cut from the lump as just described, it becomes; necessary to grind it down for purposes of microscopical investigation. For this purpose it is placed on a slip of glass, and cemented in position by Canada, balsam. Success in the operation of grinding the mounted section depends almost entirely on the way in which the mounting is done, and this in its turn depends on the condition to which the Canada has been brought.
To illustrate the operations, I will describe a specific case, viz. that of grinding the section of "gabbro"' above described, for microscopical purposes. One side of the section is probably sufficiently smooth and plane from the operation of the diamond wheel; if not, it must be ground by the finger on a slab of iron or gun-metal with emery and water, the emery passing a sieve of 80 threads to the inch. The glass base on which the section is to be mounted for grinding is placed on a bit of iron or copper plate over a Bunsen burner, and three or four drops of natural Canada balsam are placed upon it. The section is placed on the plate to heat at the same time.
The temperature must not rise so high as to cause any visible change in the Canada balsam, except a slight formation of bubbles, which rise to the surface, and can be blown off. The heating may require to be continued, say, up to twenty minutes. The progress of the operation is tested by examining the balsam as to its viscous properties.
An exceedingly simple and accurate way of testing is to dip a pair of ordinary forceps in the balsam, which may be stirred a little to secure uniformity. The forceps are introduced with the jaws in contact, and, as soon as withdrawn, the jaws are allowed to spring apart, thus drawing out a balsam thread. In a few moments the thread is cold, and if the forceps be compressed, this thread will bend.
The Canada must be heated until it is just in such a state that on bringing the jaws together the thread breaks. The forceps may open to about three-quarters of an inch. If the Canada is more viscous, so that the thread does not break, the section when cemented by it will most probably slip on the slide. On the other hand, if the balsam is more brittle, it will crumble away during the grinding.
Assuming that the proper point has been reached, the section is mounted with the usual precautions to avoid air bubbles, i.e. by dropping one edge on the balsam first. When all is cold, the surface of the section may be ground on an iron plate with emery passing the 80 sieve, till it is about 1/40 inch thick. From this point it must be reduced on ground glass by flours of emery and water; the rough particles of the former may be washed out for fine work.
The process of grinding should not take more than half an hour if the section is properly cut, etc. Beyond this point the allowable thickness must depend on the nature of the rock; a good general rule is to get the section just so thin that felspars show the yellow of the first order in a polarising, microscope. The section is then finished with, say, two minutes emery or water of Ayr-stone dust. It is better not to have the surface too smooth.
To transfer the section, the hard Canada round the sides is scraped away, and the section itself covered with some fresh Canada from the bottle. It is then warmed till it will slip off when a pin, or the invaluable dentist's chisel, is pressed against one side. If the section be very delicate, the cover slip should be placed over it before it is moved to the proper slide. The Canada used for mounting is not quite so hard as that employed in grinding, but it should be hard when cold, i.e. not sticky.
The art of preparing Canada balsam appears to consist in heating it under such conditions as will ensure its being exposed in thin layers. I have wasted a good deal of time in trying to bake Canada in evaporating basins, with the invariable result that it was either over or under-baked, and got dark in colour during the process.
On reviewing the process of rock section-cutting and mounting as just described, I cannot help thinking that, if properly systematised, it could be made much more rapid by the introduction of proper automatic grinding machinery. It also seems not improbable that a proper overhaul of available gums and cements would be found to lead to a cementing material less troublesome than Canada balsam.
§ 79. Cutting Sections of Soft Substances.
Though this art is fully treated of in books on practical biology, it is occasionally of use to the physicist, and the following note treats of that part of the subject which is not distinctly biological.
Soft materials, of which thin sections may be required, generally require to be strengthened before they are cut. For this purpose a variety of materials are available. The one most generally used is hard paraffin. The only point requiring attention is the embedding. The material must be dry.
This is accomplished by soaking in absolute alcohol, i.e. really absolute alcohol made by shaking up rectified spirit with potassium carbonate, previously dried, and then digesting for a day with large excess of quick-lime, making use of an inverted condenser and finally distilling off the alcohol without allowing it to come in contact with undried air. After soaking for some time in absolute alcohol, the material may be transferred to oil of bergamot, or oil of cloves, or almost any essential oil. After soaking in this long enough to allow the alcohol to diffuse out, the material may be lifted into a bath of melted paraffin (melting at, say, 51° C.). The process of soaking is in some cases made to go more rapidly by exhausting, and, if the material will stand it, by raising the temperature over 100° C. The soaking process may require minutes, hours, or days, according to the size and density of the material; but a few hours are usually sufficient.
When cold, the sections may be cut in any of the ordinary forms of microtome.
Another way of embedding is to soak in collodion, and then precipitate the latter in the material and around it by plunging into nearly absolute alcohol. The collodion yields a harder matrix than the paraffin.
Whatever form of cutting machine is employed, the art of sharpening the knife is the only one requiring any particular notice. The easiest way of obtaining a knife hard enough to sharpen, is to use a razor of good quality. If it has to be ground, it is best to do this on a fine Turkey stone which is conveniently rested on two bits of rubber tubing, to avoid jarring the blade. Many stones are slightly cracked, but on no account must the razor be dragged across a crack, or the edge will suffer.
The necessary and sufficient condition is that the razor must be worked in little sweeps over the stone, and pressed against the latter by little more than its own weight, and the grinding must be regular. The edge may be inspected under a microscope, and it must be perfectly smooth and even before it will cut sections. A finishing touch may be given on a leather strap, but it must be done skilfully, otherwise it is better omitted.
The necessity for providing exceptionally keen and sharp edges arose in the manufacture of phonographs, where the knife used to turn up the wax cylinders must leave a perfectly smooth surface. In 1889 this was being accomplished on an ivory lap fed with a trace of very fine diamond dust.
I have had this method in mind as a possible solution of the difficulty of razor-grinding, but have not tried it. I imagine one would use a soft steel or ivory slip rubbed over with fine diamond dust and oil by means of an agate. The lap used in the phonograph works was rotated at a high speed.
§ 80. On the Production of Quartz Threads.
[Footnote: Since this was written an article on the same subject by Mr. Boys appeared in the Electrician for 1896. The instructions therein given are in accordance with what I had written, and I have made no alteration in the text.]
In 1887 the important properties of fused quartz were discovered by Mr. Vernon Boys (Philosophical Magazine, June 1887, p. 489, "On the Production, Properties, and Some Suggested Uses of the Finest Threads"). A detailed study of the properties of quartz threads was made by Mr. Boys and communicated to the Society of Arts in 1889 (Journal of the Society of Arts, 1889). An independent study of the subject was made by the present writer in 1889 (Philosophical Magazine, July 1890, "On the Elastic Constants of Quartz Threads ").
There is also a paper in the Philosophical Magazine for 1894 (vol. xxxvii. p. 463), by Mr. Boys, on "The Attachment of Quartz Fibres." This paper also appeared in the Journal of the Physical Society at about the same date, together with an interesting discussion of the matter. In the American Journal, Electric Power, for 1894, there is a series of articles by Professor Nichols on "Galvanometers," in which a particular method of producing quartz threads is recommended. The method was originally discovered by Mr. Boys, but he seems to have made no use of it. A hunt through French and German literature on the subject has disclosed nothing of interest--nothing indeed which cannot be found in the papers mentioned.
§ 81. Quartz fibres have two great advantages over other forms of suspension when employed for any kind of torsion balance, from an ordinary more or less "astatic" galvanometer to the Cavendish apparatus. In the first place the actual strength of the fibres under longitudinal stress is remarkably high, ranging from fifty to seventy tons weight per square inch of section, and even more than this in the case of very fine threads; the second and more important point in favour of quartz depends on the wide limits within which cylindrical threads of this material obey the simplest possible law of torsion, i.e. the law that for a given thread carrying a given weight at a given temperature and having one end clamped, the twist about the axis of figure produced by a turning moment applied at the free end is proportional simply to the moment of the twisting forces, and is independent of the previous history of the thread.
It is to be noted, however, that the torsional resilience of quartz as tested by the above law is not so perfect but that our instrumental means allow us to detect its imperfections, and thus to satisfy ourselves that threads made of quartz are not things standing apart from all other materials, except in the sense that the limits within which they may be twisted without deviating in their behaviour from the law of strict proportionality by more than some unassigned small quantity, are phenomenally wide.
A torsion balance--if we except the case of certain spiral springs--is almost always called upon for information as to the magnitude of very small forces, and for this purpose it is not essential merely that some law of twisting should be exactly obeyed, but also that the resistance to twisting of the suspension should be small.
Now, regarded merely as a substance possessing elastic rigidity, quartz is markedly inferior to the majority of materials, for it is very stiff indeed; its utility depends as much as anything upon its great strength, for this allows us to, use threads of exceeding fineness. In addition to this it must be possible, and moreover readily possible, to obtain threads of uniform section over a sufficient length, or the rate of twist per unit length of the thread will vary in practice from point to point, so that the limits of allowable twist averaged over the whole thread may not be exceeded, and yet they may be greatly overpassed at particular points of the thread.
It is interesting to note that in the case of quartz we not only have a means for readily producing very uniform cylindrical threads, but that the limits of allowable rate of twist are so wide that a small departure from uniformity of section produces much less inconvenience than in the case of any other known substance.
§ 82. There are three methods generally in use for drawing quartz fibres, all depending on the fact that quartz when fused is so viscous that it may be drawn into threads of great length, without these threads breaking up into drops, or indeed without their showing any sign of doing so. The surface tension of the melted quartz must, however, be very considerable, as may be seen by examining the shape of a drop of the molten material, and this suffices to impress a rigidly cylindrical form upon the thread, the great viscosity apparently damping down all oscillation.
The first method is the one originally employed by Mr. Boys. A needle of quartz is melted somewhere in its length and is then drawn out rapidly by a light arrow, to which one end of the needle is attached, and which is projected from a kind of crossbow.
A modification of this method, which the writer has found of service when very thick threads are required, is to replace the bow and arrow by a kind of catapult.
The third method, which yields threads of almost unmanageable fineness, depends on the experimental fact that when a fine point of quartz is held in a high pressure oxygen gas blow-pipe flame, the friction of the flame gases suffices to overcome the tendency of the capillary forces to produce a spherical drop, and actually causes a fine thread to be projected outwards in the direction of the flame.
§ 83. A preliminary operation to any method is the production of a stick of fused quartz. This is managed as follows. A rock crystal or quartz pebble is selected and examined. It must be perfectly white, transparent, and free from dirt. Surface impurity can of course be got rid of by means of a grindstone. The crystal is placed in a perfectly clean Stourbridge clay crucible, furnished with a cover, and heated to bright redness for about an hour in a clean fire or in a Fletcher's gas furnace. The contents of the crucible are turned out when sufficiently cool on to a clean brick or bit of slate. It will be found that the crystal is completely broken up and the fragments must be examined in case any of them have become contaminated by the crucible, but this will not have happened if the temperature did not rise beyond a bright red heat.
The heap of fragments being found satisfactory, the next thing is to fuse some of the pieces together. Unless the preliminary heating has been efficiently carried out this will prove an annoying task, because a rock crystal generally contains so much water that it splinters under the blow-pipe in a very persistent manner. There are two ways of assembling the fragments. One is to place two tiles or bricks on edge about the heap of quartz lying upon a third tile, so that the heap occupies the angular corner or nook formed by the tiles (Fig. 64).
The oxygas blow-pipe previously described is adjusted to give its hottest flame, the bags being weighted by at least two hundredweight, if of the size described (see § 15).
The tip of the inner cone of the blow-pipe is brought to bear directly upon one of the fragments, and if the operation is performed boldly it will be found that the surface of the fragment can be fused, and the fragment thus caused to hold together before the lower side gets hot enough to suffer any contamination from the tile or brick. A second fragment may be treated in the same way, and then a third, and so on.
Finally, the fragments may be fused together slightly at the corners, and a stick may thus be formed. Of course a good deal of cracking and splitting of the fragments takes place in the process; the best pieces to operate upon are those which are well cracked to begin with, and that in such a way that the little fragments are interlocked.
An alternative method which has some advantages is to arm a pair of forceps with two stout platinum jaws, say an inch and a half long, and flattened a little at the ends. The fragments are held in these platinum forceps and the blow-pipe applied as before. This method works very well in adding to a rod which has already been partly formed, but the jaws require constant renewals. The first fragment which is fused sufficiently to cohere may also be fused to a bit of tobacco pipe, or hard glass tube or rod, and the quartz stick gradually built up by fusing fresh pieces on to the one already in position.
Fig. 64.
Since the glass or pipeclay will contaminate the quartz which has been fused on to it, it is necessary to discard the end pieces at the close of the operation. A string of fragments having been collected and stuck together, the next step is to fuse them down into a uniform rod. This is easily done by holding the string in the blow-pipe flame and allowing it to fuse down. Twisting the fused part has a good effect in assisting the operation. It is desirable to use a large jet and as powerful a flame as can be obtained during this part of the operation.
The final result should be a rod, say two or three inches long and one-eighth of an inch thick, which will in most cases contain a large number of air bubbles. Since the presence of drawn-out bubbles cannot be advantageous, it is often desirable to get rid of them, and this can conveniently be done at the present stage. The process at best is rather tedious; it consists in drawing the quartz down very fine before an intense flame, in order to allow the bubbles to get close enough to the surface to burst. A considerable loss of material invariably occurs during the process; for whenever the thin rod separates into two bits the process of flame-drawing of threads goes on, and entails a certain waste; moreover, the quartz in fine filaments is probably partially volatilised.
Sooner or later, however, a sufficient length of bubble-free quartz can be obtained. It must not be supposed that it is always necessary to eliminate bubbles as perfectly as is contemplated in the foregoing description of the treatment, but for special purposes it may be essential to do so, and in any case the reader's attention is directed to a possible source of error.
It may be mentioned in connection with this matter that crystals of quartz may look perfectly white and clear, and yet contain impurity. For instance, traces of sodium are generally present, and lithium was found in large spectroscopic quantity in five out of six samples of the purest crystals in my laboratory. The presence of lithium in rock crystal has also been detected by Tegetmeier (Vied. Ann, xli. p. 19, 1890).
After some practice in preparing rods and freeing them of bubbles the operator will notice a distinct difference in the fusibility of the samples of quartz he investigates, though all may appear equally pure to the unaided eye. It should be mentioned, however, that high infusibility cannot always be taken as a test of purity, for the most infusible, or rather most viscous, sample examined by the writer contained more lithium than some less viscous samples.
Fig. 65.
During the process of freeing the quartz from bubbles the lithium and sodium will be found to burn away, or at all events to disappear.
A rod of quartz, say three inches long, one-sixteenth of an inch in diameter, and free from bubbles for half an inch of its length, even when examined by a strong lens, is suitable for drawing into threads. The rod is manipulated exactly in the manner described under glass-blowing, and is finally drawn down at the bubble free part into a needle, say 0.02 inch in diameter (No. 25 on the Birmingham wire gauge), and 2 inches long.
Fig. 66.
There is one peculiarity about fused quartz which renders its manipulation easier than that of glass--it is impossible to break fused quartz, however suddenly it be thrust into the blow-pipe flame. A rod having a diameter of three-sixteenths of an inch--and perhaps much more--may be brought right up to the tip of the inner cone of the oxy-gas flame and held there-till one side fuses, the other being comparatively cool, without the slightest fear of precipitating a smash. In seven years' experience I have never seen a bit of once fused quartz broken by sudden heating; whether it might be done if sufficient precautions were taken I do not know.
The reason of the fortunate peculiarity of quartz in this respect is, I presume, to be found in the fact that quartz once it has been fused is really a very strong material indeed, and is also probably the least expansible substance known. From some experiments of the writer upon the subject, it may be concluded that at the most quartz which has been fused expands only about one-fifth as fast as flint-glass, at all events between 20° and 70° C.
§ 84. Drawing Quartz Threads.
The thick end of the rod of quartz is held in the fingers or occasionally in a clip. The end of the fine point is attached to a straw arrow by means of a little sealing-wax. The arrow is laid on the stock of a crossbow in the proper position for firing. See Figs. 67 and 68, which practically explain themselves.
The needle is heated by the blow-pipe till a minute length is in a state of uniform fusion; the arrow is then let fly, when it draws a thread out with it. The arrow is preferably allowed to strike a wooden target placed, say, 30 feet away from the bow, and a width of black glazed calico is laid under the line of fire to catch the thread or arrow if it falls short. The general arrangements will be obvious from the figure.
The bow is of pine in the case where very long thin threads are required, though for ordinary purposes I have found a bow of lance-wood succeed quite as well. The trigger of the bow consists of a simple pin passing through the stock and fastened at its lower end to a string connected with a board which can be depressed by foot. In the figure an ordinary trigger is shown, but the pin does just as well.
Fig 67.
The arrow is made out of about 6 inches of straw, plugged up aft by a small plug of pine or willow fastened in with sealing-wax, and projecting backwards one-eighth of an inch. This projection serves a double purpose: it gives a point of attachment for the quartz needle, and on firing the bow it forms a resisting anvil on which the string of the bow impinges. The head of the arrow is formed by a large needle stuck in with sealing-wax, and heavy enough to bring the centre of gravity of the arrow forward of one-third of its length, the condition of stability in flight.
Fig. 68.
It is not necessary to employ any feathering for these arrows; though I have occasionally used feathers or mica to "wing the shaft" no advantage has resulted therefrom.
To get fine threads a high velocity is essential. This is obtained by considering (and acting upon) the principles involved. The bow may be regarded as a doubly-tapering rod clamped at the middle. After deflection it returns towards its equilibrium position at a rate depending in general terms on the elastic forces brought into play, directly, and on the effective moment of inertia of the rod, inversely (see Rayleigh, Sound, vol. ii. chap. viii.) If the mass of the arrow is negligible compared with the bow, the rate at which the arrow moves is practically determined by that attained by the end of the bow, which is a maximum in crossing its equilibrium position.
The extent to which the arrow profits by this velocity depends on the way the bow is strung. It will be greatest when the string is perpendicular to the bow when passing its equilibrium position; or in other words, when the string is infinitely long. Since the string has mass, however, it is not permissible to make it too long, or its weight begins to make itself felt, and a point is soon reached at which the geometrical gain in string velocity is compensated for by the total loss of velocity due to the inertia of the string. In practice it is sufficient to use a string 10 per cent longer than the bow.
It is well to use a light fiddle string, served with waxed silk at the trigger catch; if this be omitted the gut gets worn through very quickly. In order to decide how far it is permissible to bend the bow, the quickest way is to make a rough experiment on a bit of the same plank from which the bow is to be cut, and then to allow a small factor of safety. In the figure the bow is of lance-wood and is more bent than would be suitable for pine.
The bow itself is tapered from the middle outwards just like any other bow. If thick threads are required, the above considerations are modified by the fact that quartz opposes a considerable resistance to drawing, and that consequently the arrow must not only have a high velocity, but a fair supply of energy as well; in other words, it must be heavy. A thin pine arrow instead of a straw generally does very well, but in this case the advantage of using pine for the bow vanishes; and in fact lance-wood does better, owing to the greater displacement which it will stand without breaking. This of course only means that a greater store of energy can be accumulated at one bending.
I had occasion to investigate whether the unavoidable spin of an arrow about its axis produces any effect on the thread, and for this purpose made arrows with inertia bars thrust through the head, i.e. an arrow with a bit of wire run through it, perpendicular to its length--forming a cross in fact--the arms of the cross being weighted at the extreme ends by shot. This form of arrow has a considerable moment of inertia about its longer axis, and consequently rotates less than a mere straw, provided that the couples tending to produce rotation are not increased by the cross arm, or the velocity too much reduced. Shooting one of these arrows slowly, I could see that it did not rotate, and when fired at a high velocity, it generally arrived at the target (placed at varying distances front bow) with the arms nearly horizontal, thus showing that it probably did not rotate much.
I did not succeed in this at the first trial, by any means. The threads got in this way were no better than those made with a single straw, whence we may conclude very provisionally that the spin of the arrow has only a small effect, if any, on the quality of the threads.
Feathering the arrow, in my experience, tends, if anything to make it spin more; for one thing, because it is practically impossible to lay the feathering on straight.
After the arrow is shot, it remains to gather in the thread, and if the latter is at all thin, we have a rather troublesome job. In a thread thirty or forty feet long, the most uniform part generally lies in the middle if the thread is thin, i.e. of the order of a ten-thousandth of an inch in diameter. If the thread is thick the most uniform part may be anywhere. The part of the thread required is generally best isolated by passing a slip of paper under it at each end and cementing the thread to the paper by means of a little paraffin or soft wax, and then cutting off the outer portions. One bit of paper may then be lifted off the calico, and the thread will carry the other bit. In this way the thread may be taken to a blackened board, where it may be mounted for stock.
By passing the two ends of the thread under a microscope, or rather by breaking bits off the two ends and examining them together, it is easy to form an Opinion as to uniformity.
Mr. Boys has employed an optical method of examining threads, but the writer has invariably found a high-power microscope more convenient and capable of giving more exact information as to the diameter of the threads.
The beginner--or indeed the practised hand--need not expect to get a thread of the exact dimensions required at the first shot. A little experience is necessary to enable one to judge of the right thickness of the needle for a thread of given diameter. The threads are so easily shot, however, that a few trials take up very little time and generally afford quite sufficient experience to enable a thread of any required diameter to be prepared.
It is no use attempting to heat an appreciable length of needle; if this be done the thread almost invariably has a thick part about the middle of its length.. It is sufficient to fuse at most about one-twentieth of an inch along the needle before firing off the bow. This can be done by means of the smaller oxygas blow-pipe jet described in the article on blow-pipes for glass-blowing, § 14. The flame must of course be turned down so as to be of a suitable size. A sufficiently small flame may be got from almost any jet.
If the needle be not equally heated all round, the thread tends to be curly; indeed by means of the catapult, threads may be pulled which, when broken, tend to coil up like the balance-springs of watches, if only care be taken to have one side of the needle much hotter than the other.
§ 85. When examining bits of threads, say thicker than the two-thousandth of an inch, under the microscope it is convenient to use a film of glycerine stained with some kind of dye, in order to render the thread more sharply visible. The thread is mounted beneath a cover slip, and a drop of the stained glycerine allowed to run in. Such a treatment gives the image of the thread a sharply defined edge 3 and the contrast between the whiteness of the thread and the colour of the background allows measurements to be made with great ease.
On the whole the easiest way of measuring the diameter of a thick thread is to use a measuring microscope, i.e. one in which the lens system can be displaced along a plane bed by means of a finely cut micrometer screw. The instruments made by the Cambridge Scientific Instrument Company do fairly well. Direct measurements up to 0.0001 inch are easily made by means of a microscope provided with a Zeiss "A" objective, and rather smaller differences of thickness can be made out by it. For thin threads the method next to be described is more fitting, because higher powers can be more conveniently used.
In this method an ordinary microscope is employed together with a scale micrometer, and either an eyepiece micrometer, or a camera and subsidiary scale. The eyepiece micrometer is the more convenient. If a camera be employed, i.e. such an one as is supplied by Zeiss, it is astonishing how the accuracy of observation may be increased by attending carefully to the illumination of both the subsidiary scale and of the thread. The two images should be as far as possible of equal brightness, and for this purpose it will be found requisite to employ small screens.
The detail of making a measurement by means of the micrometer eyepiece is very simple. The thread is arranged on the stage so as to point towards the observer, and the apparent diameter is read off on the eyepiece scale. In order to calibrate the latter it is only necessary to replace the thread by the stage micrometer, and to observe the number of stage micrometer divisions occupying the space in the eyepiece micrometer formerly occupied by the thread. It is essential that both thread and stage micrometer should occupy the same position in the field, for errors due to unequal distortion may otherwise become of importance. For this reason it is best to utilise the centre of the field only.
The same remark applies to measurements by means of the camera, where the image of the thread is projected against the reflected image of the subsidiary scale laid alongside the microscope. In this case the value of the subsidiary scale divisions must be obtained from the divisions of the stage micrometer, coinciding as nearly as possible with the position occupied by the thread. Before commencing a measurement the screens are moved about till both images appear equally bright.
Threads up to about one twenty-thousandth of an inch in diameter may be sufficiently well measured by means of a Zeiss "4 centimetre apochromatic object-glass" and an eyepiece "No. 6" with sixteen centimetre tube length. [Footnote: The objective certainly had "4 cm." marked on it, but the focal length appeared to be about I.5 mm. only.]
§ 86. Drawing Threads by the Catapult.
The bow-and-arrow method fails when threads of a greater diameter than about 0.0015 inch are required--at least if any reasonable uniformity be demanded, and no radical change in the bow and arrow be carried out.
Thus in the writer's laboratory a thread of about this diameter, within 1/10000 of an inch-13 inches long and free from air bubbles--was required. A fortnight's work by a most skilful operator only resulted in the production of two lengths satisfying the conditions.
The greatest loss of time occurs in the examination of the thread by means of the microscope.
Threads for galvanometer suspensions are conveniently from 0.0001 to 0.0004 inch in diameter, and are much more easily made and got uniform than thicker threads, to the production of which the catapult method applies.
A reference to the diagram will make the construction of the instrument quite clear. The moving end of the quartz is attached to a small boxwood slider working on a tubular girder or between wires. The quartz is secured in position by clamps shown at A and B, and motion is imparted to the slider by a stretched piece of catapult elastic (C). An easy means of regulating the pull of the elastic is to hold it back by a loop of string whose length can be varied by twisting it round a pin.
Fig. 69. [Footnote: For greater clearness of drawing, the tube carrying the slider is shown somewhat higher above the base than is convenient in practice; and the slide itself is shown too thin in the direction of the hole through it.]
Since it is not permissible to allow the slider to rebound at the end of its journey, some such arrangement of breaks as is shown must be adopted. In the diagram the bottom of the slider runs on to a brass spring between the girder and the base of the appliance, and so gets jammed; the spiral spring acts merely as an additional guard. The diagram does not show the lower spring very clearly; it is a mere strip lying in the groove.
A rod of quartz, with a needle at one end, is prepared as before and secured in the clamps. During the operation of fastening down the clamps, there is some danger of breaking the needle, and consequently it is advisable to soften the latter before and while adjusting the second clamp.
The process of drawing a thread by this method is exactly similar to the operation already described in connection with the arrow method. Though short thick threads form the product generally obtained from the catapult, it must not be supposed that thin threads cannot be obtained in this way. If a short length of a very fine needle be heated, it will be found to yield threads quite fine enough for ordinary suspension purposes, but naturally not so uniform as those obtained from the 40-foot lengths obtainable by the bow-and-arrow method.
It is easy to make spiral quartz springs resembling watch balance-springs by means of the catapult. All that is necessary is to see that the quartz is rather unequally heated before the shot is fired. In the future it is by no means impossible that such springs may have a real value, for the rigidity of quartz is known to increase as temperature rises. Hence it is probable that the springs would become stiffer as temperature rises, even though they work chiefly by bending, and little or not at all by twisting. As this is the kind of temperature variation required to compensate an uncompensated watch balance wheel, it may turn out to have some value.
§ 87. Drawing Threads by the Flame alone.
A stick of quartz is drawn down to a fine point, and the tip of this point is held in the blow-pipe flame in the position shown in Fig. 70.
Fig. 70.
The friction of the flame gases is found to be sufficient to carry forward the fused quartz and to draw it into threads in spite of the influence of the capillary forces. If a sheet of paper be suspended at a distance of two or three feet in front of the blow-pipe flame, it will be found to be covered with fine threads tangled together into a cobwebby mass. As this method is an exceedingly simple one of obtaining threads, I have endeavoured to reduce it to a systematic operation.
A sheet of cardboard, about two feet square, is painted dead black and suspended horizontally, painted side downwards (Fig. 70, A), at a height of about two feet above the blow-pipe flame. The latter is adjusted so as to point almost vertically upwards and towards the centre of the cardboard. A few half-inch pins are thrust through the card from the upper surface and pushed home; about one dozen pins scattered over the surface will be sufficient. Their object is to prevent the threads being carried away round the edge of the screen.
The flame from the jet described so often is fed from gas bags weighted to about eighty pounds per square foot of (one) surface, i.e. "4-foot" bags require from three to four hundredweight to give an advantageous pressure. [Footnote: The resulting threads were really too fine for convenient manipulation, so that unless extremely fine threads are required it will be better to reduce the pressure of the gases considerably.]
Two sticks of quartz are introduced and caused to meet just in front of the inner cone--the hottest part of the flame. They are then drawn apart so as to form a fine neck, which softens and is bent in the direction of motion of the flame gases. When fusion is complete the neck separates into two parts, and a thread is drawn from each of them. By alternately lightly touching the rods together, and drawing them apart, quite a mass of threads may be obtained in two or three minutes, when the process should be stopped. If too many threads get entangled in the pins, one gives one's self the unnecessary trouble of separating them. On taking down the card it will be found that the threads have been caught by the pins; but the card now being laid black side upwards, the former easily slip off the points.
Threads at least a foot long, and perhaps vastly longer, may be obtained by this method, and are extraordinarily fine. When I first read Professor Nichols' statement (Electric Power, 1894) as to the value of these fibres for galvanometer purposes, I was rather sceptical on the ground that the threads would tend to get annealed by being drawn gradually, instead of suddenly, from a place of intense heat to regions of lower temperature.
Now annealing threads by a Bunsen makes them rotten. The threads being immersed in the hot flame gases could only cool at the same rate as the gas, and it was not--and is not--clear to me that annealing of the threads can be avoided. On the other hand, it may be possible that a thread cooled slowly from the first does not suffer in the same way as a cold thread would do when annealed in a Bunsen flame.
Again the velocity of the gases is beyond doubt exceedingly high, so that the annealing, even supposing it to be deleterious, might not be carried very far. Threads drawn by this method and measured "dry," i.e. by mounting them on a slide without the addition of any liquid, turned out to have a diameter of about 1/20000 of an inch.
I do not think I could manage to mount such fine threads without very special trouble. All the threads lying on the board, however, were found in reality to consist of three or four separate threads, and there is no reason why several threads should not be mounted in parallel, provided, of course, that they are equally stretched and touching each other. Equality of tension in the mounting could be secured by making one attachment good, then cementing the other attachment to the other end of the threads, and "drawing" the two attachments slightly apart at the moment the cement commences to set. This method may turn out to be very valuable, for, so far as I can see, the carrying power would be increased without an increase of torsional stiffness of anything like so high an order as would be the case were one thread only employed. On the other hand, the law of torsion could hardly be quite so simple, at all events, to the second order of approximations.
§ 88. Properties of Threads.
A large number of experiments on the numerical values of the elastic constants of quartz threads have been made by Mr. Boys and his students, and by the writer. As the methods employed were quite distinct and the results wholly independent, and yet in good agreement with each other, a rounded average may be accepted with considerable confidence.
TENACITY OF QUARTZ FIBRES (BOYS).
Diameter of Thread.
Tenacity in Tons' Weight per Square Inch of Section.
Tenacity in Dynes per Square Centimetre.
Inches
Centimetres
0.00069
0.00175
51.7
8 X 109
0.00019
0.00048
74.5
11.5 X 109
Rounded mean of Boys' and Threlfall's results:
Young's Modulus at 20° C,
5.6 X 1011 C.G.S.
Modulus of Simple Rigidity at 20° C,
2.65 X 1011 C.G.S.
Modulus of Incompressibility,
1.4 X 1011 C.G.S.
Modulus of Torsion,
3.7 X 1011 C.G.S.
Approximate coefficient of linear expansion of quartz per degree between 80° C. and 30° C. is 0.0000017 (Threlfall = loc. cit.).
This must be regarded with some suspicion, as the data were not concordant. There is no doubt, however, about the extreme inexpansibility of quartz.
Temperature coefficient of modulus of torsional rigidity per degree centigrade, 22° to 98° C, 0.000133
Ditto, absolute simple rigidity, 0.000128 (Threlfall).
Limit of allowable rate of twist in round numbers is, one-third turn per centimetre, in a fibre 0.01 cm. diameter.
The limiting rate is probably roughly inversely as the diameter.
Attention must be called to the rapid increase in the torsional rigidity of these threads as the temperature rises. A quartz spiral spring-balance will be appreciably stronger in hot weather.
§ 89. In the majority of instances in which quartz threads are applied in the laboratory, it is desirable to keep the coefficient of torsion as small as possible, and hence threads are used as fine as possible.
It is convenient to remember that a thread 0.0014 cm. or 0.0007 inch in diameter breaks with a weight of about ten grammes, and may conveniently be employed to carry, say, five grammes. With threads three times finer the breaking strength per unit area increases, say, 50 per cent. In ordinary practice--galvanometric work for instance--where it is desirable to use a thread as fine and short as possible to sustain a weight up to, say, half a gramme, it will be found that fibres five centimetres long or over give no trouble through defect of elastic properties. A factor of safety of two is a fair allowance when loading threads.
No difficulty will be experienced in mounting threads having a diameter of 0.0002 inch or over. With finer threads it is necessary to employ very dark backgrounds (Mr. Boys uses the darkness of a slightly opened drawer), or the threads cannot be sufficiently well seen.
In the case of instruments in which threads remain highly twisted for long periods of time, the above rule as to the safe limit of twist does not allow of a sufficient margin; it is only applicable to galvanometric and similar purposes.
The cause of the increase in tenacity as the diameter diminishes is at present unknown. It is due neither to an effect of annealing (annealed threads are rotten), nor is it a skin effect, nor is it due to the cooling of the thread under higher capillary pressure. It is, however, possible that it may be associated with some kind of permanent set taken by the fibres during the stage of passage from the liquid to the solid state.
§ 90. On the Attachment of Quartz Fibres.
For many purposes it is sufficient to cement the fibres in position by means of ordinary yellow shellac, but where very great accuracy is aimed at, the shellac (being itself imperfectly elastic and exposed to shearing stress) imposes its imperfections on the whole system. This source of error can be got over by soldering the threads in position. Attempts were made by the writer in this direction, with fair success, in 1889, but as Mr. Boys has carried the art to a high degree of perfection, I will suppress the description of my own method and describe his in preference. It has, of course, been frequently repeated in my laboratory.
In many cases, however, if not in all, it may be replaced by Margot soldering, as already described, a note on the application of which to this purpose will follow.
A thread of the proper diameter having been selected, it is cut to the right length. With fine threads this is not always a perfectly easy matter. The best way is for the operator to station himself facing a good light, not sunlight, which is too tiring to the eye, but bright diffused light. The thread will be furnished with bits of paper stuck on with paraffin at both ends, as already described.
A rough sketch of the apparatus--or, at all events, two lines showing the exact length which the free part of the thread must have--are marked on a smooth board, and this is supported with its plane vertical. The thread is held against the board, and the upper piece of paper is stuck lightly to the board with a trace of soft wax, so that the lower edge of the paper is at any desired height above the upper mark. This distance is measured, and forms the length of thread allowed to overlap the support. A second bit of paper is attached below the lower mark, a margin for the attachment of the lower end being measured and left as before. The thread will be most easily seen if the board is painted a dead black.
If it is desired to attach the thread to its supports merely by shellac, this is practically all that needs to be done. The supports should resemble large pins. The upper support will be a brass wire in most cases, and will require to be filed away as shown in the sketch (Fig. 71). It is then coated with shellac by heating and rubbing upon the shellac. As previously noted, the shellac must not be overheated.
The thread is cut off below the lower slip of paper, and the upper support being conveniently laid in a horizontal position on another dead-black surface, the thread is carried to it and laid as designed against the shellac, which is now cold. When the thread is in place, a soldering iron is put against the brass wire, and the shellac gradually melted till it closes over the thread.
Fig. 71.
The iron is then withdrawn and the thread pulled away from the point for one-twentieth of an inch or less. This ensures that the thread makes proper contact with the cement, and also that it is free from kinks; of course, it must leave the cement in the proper direction. A similar process is next carried out with respect to the lower attachment, and the ends of the thread are neatly trimmed off.
Both ends of the thread being secured, the next step is to transfer the upper support to a clip stand, the suspended parts being held by hand, so that the weight comes on the thread very gradually. In this way it will be easily seen whether the thread is bent where it enters the shellac, and should this be the case, a hot iron must be brought up to the shellac and the error rectified.
When both the support and the suspended parts are brought nearly to the required bearing, the hot iron is held for a moment close up to each attachment, the hand being held close below but not touching the suspended parts, and both attachments are allowed to straighten themselves out naturally.
These details may appear tiresome, and so they are when written out at length, but the time occupied in carrying them out is very short, and quartz threads break easily, unless the pull upon them is accurately in the direction of their length at all points.
In the event of its being decided to attach the thread by soldering, the process is rather more expensive in time, but not otherwise more troublesome.
Fig. 72. Fig. 73.
The thread being cut as before to the proper length, little bits of aluminium foil are smeared all over with melted shellac and suspended from the thread replacing the paper slips before described. It is important that no paraffin should be allowed to touch the thread anywhere near a point intended to be soldered. The thread is hung up from a clip stand by one of the bits of foil, and the lower end is washed by dipping it into strong nitric acid for a moment and thence into water. The object of smearing the foils all over with shellac is to prevent them being acted upon by the acid. The threads are not very easily washed acid free, but the process may be assisted by means of a fine camel's-hair pencil.
Some silvering solution made as described (§ 65) is put into a test tube; the thread, after rinsing with distilled water, is lowered into the solution so far as is required, and is allowed to receive a coating of silver. It has been observed that the coating of silver must not be too thick--not sufficiently thick to be opaque. A watch may be kept on the process by immersing a minute strip of mica alongside the thread.
The silvered thread is rinsed with distilled water and allowed to dry.
Meanwhile the other end of the thread may be silvered. When both ends are silvered the process of coppering by electro deposit is commenced. A test tube is partially filled with a ten per cent solution of sulphate of copper, and several copper wires are dipped into it to form an anode. The thread is lowered carefully into the solution so as not to introduce air bubbles, and the silvered part is allowed to project far enough above the surface of the solution to come in contact with a fine copper wire. The circuit is closed through a Leclanché cell and a resistance box.
It is as well to begin with a fair resistance, say 100 ohms out in the box, and the progress of the deposit is watched by means of a low-power microscope set up in front of the thread. If the copper appears to come down in a granular form, the resistance is too small and must be increased; if no headway appears to be made, the resistance must be diminished.
As soon as a fair coat of copper has come down, i.e. when the diameter of the thread is about doubled, the process is interrupted. The thread is withdrawn, washed, dipped in a solution of chloride of zinc, and carefully tinned by dragging it over a small clean drop of solder on a soldering bit.
During this part of the process the shellac is apt to get melted if the iron is held too close, so that it is advisable to begin by making the thread somewhat over long. The end of the thread must only be trimmed off at the conclusion of the operation, i.e. after the thread is soldered up. The thread is attached to the previously tinned supports much in the same way as has been described under the head of shellac attachments. It does not very much matter whether both ends are coppered before one is soldered up or not. At the conclusion of the whole process the superfluous copper and silver are dissolved off by a little hot strong nitric acid applied on a glass hair pencil. This is best done by holding the thread horizontally with the assistance of clip stands.
If the thread is too delicate to bear brushing, the nitric acid may be applied by pouring out a big drop into a bit of platinum foil and holding this below the thread so as to touch it lightly. The dissolving of the copper and silver is, of course, followed by copious washing with hot water. This process is more laborious than might be imagined, but it may be shortened by heating the platinum foil supporting the water (Fig. 74).
Fig. 74
The washing part of the process is, in the opinion of the writer, the most difficult part of the whole business, and it requires to be very thorough, or the thread will end by drawing out of the solder. In many cases it is better to try to do without any application of nitric acid at all, but, of course, this involves silvering and coppering to exact distances from the ends of the thread--at all events, in apparatus where the effective length of the thread is narrowly prescribed.
It is important not to leave the active parts of the thread appreciably silvered, for the sake of avoiding zero changes due to the imperfect elasticity of the silver. In this soldering process ordinary tinman's solder may be employed; it must be applied very free from dust or oxide.
§ 91. Other Modes of soldering Quartz.
Thick rods of quartz may be treated for attachment by solder in the same way as glass was treated by Professor Kundt to get a foundation for his electrolytically deposited prisms. [Footnote: See Appendix at end of book.]
The application of a drop of a strong solution of platinum tetrachloride to the rod will, on drying, give rise to a film of the dry salt, and this may be reduced in the luminous gas flame. During the process, however, the quartz is apt to get rotten, especially if the temperature has been anything approaching a full red heat. The resulting platinum deposit adheres very strongly to the quartz, and may be soldered to as before. This method has been employed by the writer with success since 1887, and may even be extended to thick threads.
It was also found that fusible metal either stuck to or contracted upon clean quartz so as to make a firm joint. In the light of M. Margot's researches (already described), it occurred to me that perhaps my experience was only a special case of the phenomena of adhesion investigated with so much success by M. Margot. I therefore tried whether the alloy of tin and zinc used for soldering aluminium would stick to quartz, and instantly found that this was indeed the case.
Adhesion between the alloy and perfectly clean quartz takes place almost without rubbing. A rod of quartz thus "tinned" can be soldered up to anything to which solder will stick, at once. On applying the method to thick quartz threads, success was instantaneous (the threads were some preserved for ordinary galvanometer suspensions); but when the method was applied to very fine threads, great difficulty in tinning the threads was experienced. The operation is best performed by having the alloy on the end of an aluminium soldering bit, and taking care that it is perfectly free from oxide before the thread is drawn across it. There was no difficulty in soldering a thread "tinned" in this manner to a copper wire with tinman's solder, and the joint appeared perfect, the thread breaking finally at about an inch away from the joint.
I allow Mr. Boys' method to stand as I have written it, simply because I have not had time as yet to make thorough tests of the durability of "Margot" joints on the finest threads; but I have practically no doubt as to its perfect applicability, provided always that the solder can be got clean enough when melted on the bit. Very fine threads will require to be stretched before tinning, in order to enable them to break through the capillary barrier of the surface of the melted solder.
§ 92. Soldering.
It is almost unfair to the arts of the glass-blower or optician to describe them side by side with the humble trade of soldering. Nevertheless, no accomplishment of a mechanical kind is so serviceable to the physicist as handiness with the soldering bit; and, as a rule, there is no other exercise in which the average student shows such lamentable incapacity. The following remarks on the subject are therefore addressed to persons presumably quite ignorant of the way in which soldering is carried out, and do not profess to be more than of the most elementary character.
For laboratory purposes three kinds of solder are in general sufficient. One is the ordinary tinman's solder composed of lead and tin. The second is "spelter," or soft fusible brass, and the third is an alloy of silver and brass called silver solder.
Tinman's solder is used for most purposes where high temperatures are not required, or where the apparatus is intended to be temporary. The "spelter," which is really only finely granulated fusible brass, is used for brazing iron joints. The silver solder is convenient for most purposes where permanency is required, and is especially suited to the joining of small objects.
§ 93. Soft tinman's solder is made by melting together two parts of grain tin and one of soft lead--the exact proportions are not of consequence--but, on the other hand, the purer the constituents the better the solder. Within certain limits, the greater the proportion of tin the cleaner and more fusible is the solder. It is usually worth while to prepare the solder in the laboratory, for in this way a uniform and dependable product is assured. Good soft lead is melted in an iron ladle and skimmed; the temperature is allowed to rise very little above the melting-point. The tin is then added little by little, the alloy stirred vigorously and skimmed, and sticks of solder conveniently cast by sweeping the ladle over a clean iron plate, so as to pour out a thin stream of solder. If the solder be properly made it will have a mat and bright mottled surface, and will "crackle" when held up to the ear and bent.
Perhaps the chief precaution necessary in making solder is to exclude zinc. The presence of a very small percentage of this metal entirely spoils the solder for tinman's work by preventing its "running" or flowing smoothly under the soldering bit.
Fig. 75.
Fig. 76.
Fig. 77.
§ 94. Preparing a Soldering Bit.
The wedge-shaped edge of one of the forms of bit shown in the sketch is filed to shape and the bit heated in a fire or on a gas heater. A bit of rough sandstone, or even a clean soft brick, or a bit of tin plate having some sand sprinkled over it, is placed in a convenient position and sprinkled with resin.
As soon as the bit is hot enough to melt solder it is withdrawn and a few drops of solder melted on to the brick or its equivalent. The iron or bit is then rubbed to and fro over the solder and resin till the former adheres to and tins the copper head. It will be found advisable to tin every side of the point of the bit and to carry the tinning back at least half an inch from the edge.
If the solder obstinately refuses to adhere, the cause is to be sought in the oxidation of the copper, or of the solder, or both--in either case the result of too high a temperature or too prolonged heating. The simple remedy is to get the iron hot, and then to dress it with an old file, so as to expose a bright surface, which is instantly passed over the resin as a means of preserving it from oxidation. If the process above described be now carried out, it will be found that the difficulty disappears.
Before using the iron, wipe off any soot or coke or burned resin by means of an old rag. An iron tinned in this way is much to be preferred to one tinned by means of chloride of zinc.
A shorter and more usual method is carried out as follows: The solution of chloride of zinc is prepared by adding bits of zinc to some commercial hydrochloric acid diluted with a little (say 25 per cent) of water. The acid may conveniently be placed in a small glazed white jar (a jam pot does excellently), and this should only be filled to about one-quarter of its capacity. An excess of zinc may be added.
It may be fancy, but I prefer a soldering solution made in this way to a solution of chloride of zinc bought as a chemical product. The jar is generally mounted on a heavy leaden base, so as to avoid any danger of its getting knocked over, for nothing is so nasty or bad for tools as a bench on which this noxious liquid has been upset (Fig. 78).
Fig. 78.
To tin a soldering bit, a little of the fluid is dipped out of the jar on to a bit of tin plate bent up at the edges--a few drops is sufficient--and the iron is heated and rubbed about in the liquid with a drop of solder. If the iron is anything like clean it will tin at once and exhibit a very bright surface, but quite dirty copper may be tinned by dipping it for a moment in the liquid in the pot and then working it about over the solder. An iron so tinned remains covered with chloride of zinc, and this must be carefully wiped off if it is intended to use the iron with a resin or tallow flux in lead soldering.
One disadvantage of this process is that the copper bit soon gets eaten into holes and requires to be dressed up afresh. On the other hand, an iron so tinned always presents a nice clean solder surface until the next time it is heated, when it generally becomes very dirty and requires to be carefully wiped before using.
In my experience also an iron so tinned is more easily spoiled as to the state of its surface, "detinned," in fact, by overheating than when the tinning is carried out by resin and friction. When this happens, the shortest way out of the difficulty is the application of the old file so as to obtain a perfectly fresh surface. No one who knows his business ever uses an iron that is not perfectly clean and well tinned.
The iron may be cleaned from time to time by heating it red hot and quenching it in water to get rid of the oxide, which scales off in the process.
§ 95. Soft Soldering.
In the laboratory the chief application of the process is to copper soldering during the construction of electrical apparatus and to zinc soldering for general purposes.
In ninety-nine cases out of every hundred where difficulties occur their origin is to be traced to dirt. There seems to be some inexplicable kink in the human mind which renders it callous to repeated proofs of the necessity for cleaning surfaces which it is intended to solder. The slightest trace of albuminous or gelatinous matter or shellac will prevent solder adhering to most metals and the same remark applies in a measure to the presence of oxides, although these may be removed by chloride of zinc or prevented from forming by resin or tallow. A touch with an ordinarily dirty hand--I refer to a solderer's hand--will often soil work sufficiently to make the adherence of solder difficult.
The fluxes most generally employed are tallow for lead, resin or Venice turpentine for copper, chloride of zinc for anything except lead, which never requires it. The latter flux has the property (also possessed by borax at a red heat) of dissolving any traces of oxide which may be formed, as well as acting as a protecting layer to the metal.
We may now turn to the consideration of a simple case of soldering, say the joining of two copper wires. The wires are first cleaned either by dipping in a bath of sulphuric and nitric acids--a thing no laboratory should be without--or by any suitable mechanical means. The cleaned wires are then twisted together--there is a regulation way of doing this, but it presents no advantage in laboratory practice--and the joint is sprinkled over with resin, or painted with a solution of resin in alcohol.
The iron, being heated and floated with solder, is held against the joint, the latter being supported on a brick, and the solder is allowed to "sweat" into the joint. Enough solder must be present to penetrate right through the joint. Nothing is gained by rubbing violently with the iron. If the copper is clean it will tin, and if it is dirty it won't, and there the matter ends.
Beginners generally use too small or too cold a bit, and produce a ragged, dirty joint in consequence. If the saving of time be an object, the joint may be twisted together on ordinarily dirty oxidised wires and heated to, say, 200° C. It is then painted with chloride of zinc and soldered with the bit.
There is a difference of opinion as to the relative merits of chloride of zinc and of resin as a flux in soldering copper. Thus the standing German practice is, or was, to employ the former flux in every case for soldering electric light wires, while in England the custom used to be to specify that soldering should be done by resin, and this custom may still prevail; it lingers in Australia at all events.
However, it is agreed on all hands that when chloride of zinc is used it must be carefully washed off. I have known of an electrical engineer insisting on his workmen "licking" joints with their tongues to ensure the total removal of chloride of zinc; it has a horrible taste; and I have occasionally pursued the same plan myself when the soldering of fine wires was in question.
In any case, it is very certain that chloride of zinc left in a joint will ruin it sooner or later by loosening the contact between copper and solder.
Very often it is requisite to solder together two extensive flat surfaces--for instance, in "chucking" certain kinds of brass work. The surfaces to be soldered must be carefully tinned, most conveniently by the help of the blow-pipe and chloride of zinc. After tinning, the surfaces are laid together and heated so as to "sweat" them together; the phrase, though inelegant, is expressive.
96. Soldering Tin Plate.
If the plate be new and clean, a little resin or its solution in alcohol is all that is necessary as a flux. If the tin plate is rusty the rust must be removed and the clean iron, or rather mild steel, surface exposed. The use of chloride of zinc is practically essential in this case. Tin plate is often spotted with rust long before it becomes rusty as a whole, when, of course, it may be regarded as worn out, and such rust spots are most conveniently removed by means of the plumber's shave-hook. The shave-hook is merely a peculiarly shaped hard steel scraping knife on a handle (Fig. 79).
Fig. 79
With tin plate the soldering of long joints is often necessary. The plate must be temporarily held in position either by binding with iron wire, fastening by clamps, or holding by an assistant. The flux is applied and the iron run slowly along the joint. Enough solder is used to completely float the tip of the iron. By arranging the joint so that it slopes downward slightly, and commencing at the upper end, the solder may be caused to flow after the iron, and will leave a joint with the minimum permissible amount of solder in it. By regulating the slope, heat of iron, etc, any desired quantity of solder may be run into the joint.
§ 97. Soldering Zinc.
Zinc alloys with soft solder very easily, and by so doing entirely spoils it, making, it "crumbly," dirty, and preventing it running. Consequently, in soldering up zinc great care must be taken to prevent the solder becoming appreciably contaminated by the zinc. To this end the zinc surfaces are cleaned by means of a little hydrochloric acid, which is painted on instead of chloride of zinc. Plenty of solder is melted on to the work, and is drawn along over the joint by a single slow motion of the soldering bit. The iron must be just hot enough to make the solder flow freely, and it must never be rubbed violently on the zinc or allowed to linger in one spot; the result of the latter action will be to melt a hole through the zinc, owing to the tendency of this metal to form an easily fusible alloy with the solder.
The art of soldering zinc is a very useful one in the laboratory. The majority of physicists appear to overlook the advantages of zinc considered as a material for apparatus construction. It is light, fairly strong, cheap, easily fusible, and yet hard and elastic when cold. It may be worked as easily as lead at a temperature of, say, 150° to 200° C, and slightly below the melting-point (423° C.) it is brittle and may & powdered. The property of softening at a moderate temperature is invaluable as a means of flattening zinc plate or shaping it in any way. During the work it may be held by means of an old cloth. Zinc sheet which has been heated between iron plates and flattened by pressure retains its flatness very fairly well after cooling.
§ 98. Soldering other Metals.
Iron.
The iron must be filed clean and then brushed with chloride of zinc solution. Some people add a little sat ammoniac to the chloride of zinc, but the improvement thus made is practically inappreciable. If the iron is clean it tins quite easily, and the process of soldering it is perfectly easy and requires no special comment.
Brass.
The same method as described for iron succeeds perfectly. The brass, if not exceedingly dirty, may be cleaned by heating to the temperature at which solder melts (below 200° C.), and painting it over with chloride of zinc, or dipping it in the liquor. If now the brass be heated again in the blow-pipe flame, it will be found to tin perfectly well when rubbed over with solder.
German Silver, Platinoid, Silver, and Platinum are treated like iron. With regard to silver and platinum the same precautions as recommended in the case of zinc must be observed, for both these metals form fusible alloys with solder.
Gold when pure requires no flux. Standard gold, which contains copper, solders better with a little chloride of zinc.
Lead must be pared absolutely clean and then soldered quickly with a hot iron, using tallow as a flux. Since solder if over hot will adhere to lead almost anywhere, plumbers are in the habit of specially soiling those parts to which it is not intended that solder shall adhere. The "soiling" paint consists of very thin glue, called size, mixed with lampblack; on an emergency a raw potato may be cut in half, and the work to be soiled may be rubbed over with the cut surface of the potato.
Hard Carbon or gas coke may be soldered after coating with copper by an electrolytic process, as will be described.
§ 99. Brazing.
Soldering at a red heat by means of spelter is called brazing. Spelter is soft brass, and is generally made from zinc one part, copper one part; an alloy easily granulated at a red heat; it is purchased in the granular form.
The art of brazing is applied to metals which will withstand a red heat, and the joints so soldered have the strength of brass.
The pieces to be jointed by this method must be carefully cleaned and held in their proper relative positions by means of iron wire. It is generally necessary to soften iron wire as purchased by heating it red hot and allowing it to cool in the air; if this is not done the wire is usually too hard to be employed satisfactorily for binding.
Very thin wire--i.e. above No. 20 on the Birmingham wire gauge--does not do, for it gets burned through, and perhaps allows the work to fall apart at a critical moment.
The work being securely fastened, the next step is to cover the cleaned parts with flux in order to prevent oxidation. For this purpose "glass borax" is employed. "Glass" borax is simply ordinary borax which has been fused for the purpose of getting rid of water of crystallisation. The glass borax is reduced to powder in an iron mortar, for it is very hard, and is then made up into a cream with a little water. This cream is painted on to the parts of the work which are destined to receive the solder.
The next step is to prepare the spelter, and this is easily done by mixing it with the cream, taking care to stir thoroughly with a flattened iron wire till each particle of spelter is perfectly covered with the borax. The mixture should not be too wet to behave as a granular mass, and may then be lifted on to the work by means of the iron spatula.
Care must be taken to place the spelter on those parts only which are intended to receive it, and when this is done, the joint may be lightly powdered over with the dry borax, and will then be ready for heating.
If the object is of considerable size it is most conveniently heated on the forge; if small the blowpipe is more convenient. In the latter case, place the work on a firebrick, and arrange two other bricks on edge about it, so that it lies more or less in a corner. A few bits of coke may also be placed on and about the work to increase the temperature by their combustion, and to concentrate the flame and prevent radiation. The temperature is gradually raised to a bright red heat, when the spelter will be observed to fuse or "run," as it is technically said to do.
If the cleaning and distribution of flux has been successful, the spelter will "run" along the joint very freely, and the work should be tapped gently to make sure that the spelter has really run into the joint. The heating may be interrupted when the spelter is observed to have melted into a continuous mass. As soon as the work has fallen below a red heat it may be plunged into water, a process which has the effect of cracking off the glass-like layer of borax.
There is, however, some risk of causing the work to buckle by this violent treatment, which must of course be modified so as to suit the circumstances of the case. If the joint is in such a position that the borax cannot be filed off, a very convenient instrument for its removal by scraping is the watchmaker's graver, a square rod of hard steel ground to a bevelled point (Fig. 80).
Fig. 80.
Several precautions require to be mentioned. In the first place, spelter is merely rather soft brass, and consequently it often cannot be fused without endangering the rest of the work. A good protection is a layer of fireclay laid upon the more delicate parts, such for instance as any screwed part.
Gun-metal and tap-metal do not lend themselves to brazing so readily as iron or yellow brass, and are usually more conveniently treated by means of silver solder.
Spelter tends to run very freely when it melts, and if the brass surface in the neighbourhood of the joint is at all clean, may run where it is not wanted. Of course some control may be exercised by "soiling" with fireclay or using an oxidising flame; but the erratic behaviour of spelter in this respect is the greatest drawback to its use in apparatus construction. The secret of success in brazing lies in properly cleaning up the work to begin with, and in disposing the borax so as to prevent subsequent oxidation.
§ 100. Silver Soldering.
This process resembles that last described, but instead of spelter an alloy of silver, copper, and zinc is employed. The solder, as prepared by jewellers to meet special cases, varies a good deal in composition, but for the laboratory the usual proportions are:
For soft silver solder
Fine silver 2 parts Brass wire 1 part
For hard silver solder
Sterling silver 3 parts Brass wire 1 part
The latter is, perhaps, generally the more convenient.
Silver solders may, of course, be purchased at watchmakers' supply shops, and as thus obtained, are generally in thin sheet. This is snipped fine with a pair of shears preparatory to use.
As odds and ends of silver (from old anodes and silver residues) generally accumulate in the laboratory, it is often more convenient to make the solder one's self. In this case it must be remembered in making hard solder by the second receipt that standard silver contains about one-twelfth of its weight of copper--exactly 18 parts copper to 220 silver.
The silver is first melted in a plumbago crucible in a small furnace together with a little borax; if any copper is required this is then added, and finally the brass is introduced. When fusion is complete, the contents of the crucible are poured into any suitable mould.
The quickest and most convenient way of preparing the alloy for use is to convert it into filings with the assistance of a coarse file, or by milling it, if a milling machine is available.
Equal volumes of filings and powdered glass borax are made into a thin paste with water, and applied in an exactly similar manner to that described under the head of "brazing." In fact all the processes there described may be applied equally to the case under discussion, the substitution of silver for spelter being the only variation.
The silver solder is more manageable than spelter, and does not tend to run wild over the work: a property which makes it much more convenient both for delicate joints and in cases where it is desired to restrict the solder to a single point or line. Small objects are almost invariably soldered with silver solder, and are held by forceps or on charcoal in the pointed flame of an ordinary blow-pipe.
§ 101. On the Construction of Electrical Apparatus: Insulators.
It is not intended to deal in any way with the design of special examples of electrical apparatus, but merely to describe a rather miscellaneous set of materials and processes constantly required in its construction.
It is not known whether there is such a thing as a perfect insulator, even if we presuppose ideal circumstances. Materials as they exist must be regarded merely as of high specific resistance, that is if we allow ourselves to use such a term in connection with substances, conduction through which is neither independent of electromotive force per unit length, nor of previous history.
Even the best of these substances generally get coated with a layer of moisture when exposed to the air, and this as a rule conducts fairly well. Very pure crystalline sulphur and fused quartz suffer from this defect less than any other substances with which the writer is acquainted, but even with them the surface conductivity soon grows to such an extent as totally to mask the internal conduction.
It is proposed to give a brief account of the properties of some insulating substances and their application in electrical construction, and at the same time to indicate the appliances and methods requisite for working them.
With regard to the specific resistances which will be quoted, the numbers must not be taken to mean too much, partly for the reason already given. It is also in general doubtful whether sufficient care has been taken to distinguish the body from the surface conductivity, and consequently numerical estimates are to be regarded with suspicion. The question of "sampling" also arises, for it must be remembered that a change in composition amounting to, say, 1/10000 per cent may be accompanied by a million-fold change in specific resistance.
§ 102. Sulphur.
This element exists in several allotropic forms, which have very different electric properties. After melting at about 125° C, and annealing at 110° for several hours, the soluble crystalline modification is formed. After keeping for some days--especially if exposed to light--the crystals lose their optical properties, but remain of the same melting-point, and are perfectly soluble in carbon bisulphide. The change is accompanied by a change in colour, or rather in brightness, as the transparency changes.
The "specific resistance" of sulphur in this condition is above 1028 C.G.S.E.M. units, or 1013 megohms per cubic centimetre for an electric intensity of say 12,000 volts per centimetre. This is at ordinary temperatures. At 75° C. the specific resistance falls to about 1025 under similar conditions as to voltage.
In all cases the conductivity appears to increase with the electric intensity, or at all events with an increase in voltage, the thickness of the layer of sulphur remaining the same.
The specific inductive capacity is 3.162 at ordinary temperatures, and increases very slightly with rise of temperature. [Footnote: March 1897.--It is now the opinion of the writer that though the specific inductive capacity of a given sample of a solid element is perfectly definite, yet it is very difficult to obtain two samples having exactly the same value for this constant, even in the case of a material so well defined as sulphur.]
The total residual charge, after ten minutes' charging with an intensity of 12,000 volts per centimetre, is not more than 4 parts in 10,000 of the original charge. In making this measurement the discharge occupied a fraction of a second. The electric strength for a homogeneous plate of crystalline sulphur is not less than 33,000 volts per centimetre, and probably a good deal more. If the sulphur is contaminated with up to 3 per cent of the amorphous variety, as is the case if it is cooled fairly quickly from a temperature of 170° C. or over, the specific resistance falls to from 10^25 to 10^26 at ordinary temperatures; and the specific inductive capacity increases up to 3.75, according to the amount of insoluble sulphur present.
The residual charge under circumstances similar to those described above, but with an intensity of about 4000 volts per centimetre is, say, 2 per cent of the initial charge. So far as the writer is aware sulphur is the only solid non-conductor which can be easily obtained in a condition of approximate purity and in samples sufficiently exactly comparable with one another; it is the only one, therefore, that repays any detail of description.
Very pure sulphur can be bought by the ton if necessary from the United Alkali Company of Newcastle-on-Tyne. It is recovered from sulphur waste by the Chance process, which consists in converting the sulphur into hydrogen sulphide, and burning the latter with insufficient air for complete combustion. The sulphur is thrown out of combination, and forms a crystalline mass on the walls and floor of the chamber.
The sulphur which comes into the market consists of this mass broken up into convenient fragments. In order to purify it sufficiently for use as an insulator, the sulphur may be melted at a temperature of 120° to 140° C, and filtered through a plug of glass wool in a zinc funnel; as thus prepared it is an excellent insulator. To obtain the results mentioned in the table it is, however, necessary to conduct a further purification (chiefly from water) by distillation in a glass retort.
The sulphur thus obtained may be cast of any desired form in zinc moulds, the castings and moulds being immediately removed to an annealing oven at a temperature of from 100° to 110° C, where they are left for several hours. If the sulphur is kept melted for some time at 125° C. the annealing is not so important.
The castings may be removed from the mould by slightly heating the latter, but many breakages result. Insulators made on this plan are much less affected by the condensation of moisture from the air than anything except fused quartz. They are, however, very weak mechanically, and apt to crack by exposure to such changes of temperature as go on from day to day. It is clear, however, that in spite of this their magnificent electrical properties fit them for many important uses.
If the sulphur be cooled rapidly from 170° C. or over, a mixture of the crystalline and amorphous varieties of sulphur is obtained. This mixture is very much stronger and tougher than the purely crystalline substance, and may be worked with ordinary hardwood tools into fairly permanent plates, rods, etc. Sheets of pure thick filter paper may also be dipped into sulphur at 170° C, at which temperature air and moisture are mostly expelled, and such sheets show a very considerable insulating power. The sulphur does not penetrate the paper, which therefore merely forms a nucleus.
Cakes of the crystalline or mixed varieties may be made by grinding up some purified sulphur, moistening it with redistilled carbon bisulphide, or toluene, or even benzene (C6H6), and pressing it in a suitable mould under the hydraulic press. The plates thus formed are porous, but are splendid insulators, especially if made from the crystalline variety of sulphur, and they appear to keep their shape very well, and do not crack with ordinary temperature changes.
The metals which resist the action of sulphur best are gold and aluminium; while platinum and zinc are practically unacted upon at temperatures below a red heat--in the former case,--and below the boiling-point of sulphur in the latter.
A very convenient test of the purity of sulphur is the colour assumed by it when suddenly cooled from the temperature at which it is viscous. Quite pure sulphur remains of a pale lemon yellow under this treatment, but the slightest trace of impurity, such as arises from dust containing organic matter, stains the sulphur, and renders it darker in colour.
§ 103. Fused Quartz.
This is on the whole the most reliable and most perfect insulator for general purposes. No exact numerical data have been obtained, but the resistivity must certainly be of the same order as that of pure sulphur at its best. The influence of the moisture of the air also reaches its minimum in the case of quartz, as was originally observed by Boys.
As yet, however, the material can only be obtained in the form of rods or threads. For most purposes rods of about one-eighth of an inch in diameter are the most convenient. These rods may be used as insulating supports, and succeed perfectly even if they interpose less than an inch of their length to electrical conduction. The sketch (Figs. 81 and 81A) shows (to a scale of about one-quarter full size) a complete outfit for elementary electrostatic experiments, such as has been in use in the writer's laboratory for five years. With these appliances all the fundamental experiments may be performed, and the apparatus is always ready at a moment's notice.
Fig. 81.
Though quartz does not condense moisture or gas to form a conducting layer of anything like the same conductivity as in the case of glass or ebonite, still it is well to heat it if the best results are to be obtained. For this purpose a small pointed blow-pipe flame may be used, and the rods may be got red-hot without the slightest danger of breaking them. They then remain perfectly good and satisfactory for several hours at least, even when exposed to damp and dusty air.
The rods are conveniently held in position by small brass ferrules, into which they are fastened by a little plaster of Paris. Sealing-wax must be avoided, on account of the inconvenience it causes when the heating of the rods is being carried out.
One useful application of fused quartz is to the insulation of galvanometer coils (Fig. 82), another to the manufacture of highly insulating keys (Fig. 83); while as an insulating suspension it has all the virtues. If it is desired to render the threads conducting they may be lightly silvered, and will be found to conduct well enough for electrometer work before the silver coating is thick enough to sensibly impair their elastic properties.
Fig. 81A.
Fig. 82 is a full-size working drawing of a particular form of mounting for galvanometer coils. The objects sought to be attained are: (1) high insulation of the coils,
(2) easy adjustment of the coils to the suspended system.
The first object is attained as follows. The ebonite ring A is bored with four radial holes, through which are slipped from the inside the fused quartz bolt-headed pins B. The coil already soaked in hard paraffin is placed concentrically in the ring A by means of a special temporary centering stand. The space between the coil and the ring is filled up with hard paraffin, and this holds the quartz pins in position. The system of ebonite ring, coil, and pins is then fastened into the gun-metal coil carrier, which is cut away entirely, except near the edges, where it carries the pin brackets C. These brackets can swivel about the lower fastening at E before the latter is tightened up.
The coil is now adjusted in the adjusting stand to be concentric with the axis of symmetry of the coil carrier, and the supporting pins are slipped into slot holes cut in the brackets, the brackets being swivelled as much as necessary to allow of this. When the pins are all inserted the brackets are screwed up by the screws at E. The pins are then cemented firmly to the brackets by a little plaster of Paris. The coil carrier can now be adjusted to the galvanometer frame by means of screws at D, which pass through wide holes in the carrier and bold the latter in position by their heads. In the sectional plan the parts of the galvanometer frame are shown shaded. The front of the frame at F F is of glass, and the back of the frame is also made of glass, though this is not shown in the section.
A represents an ebonite ring into which the wire coil is cemented by means of paraffin. B B B B are quartz pins, with heads inside the ebonite ring. C C C are slotted brackets adjustable to the pins and capable of rotation by releasing the screws E E. D D are the screws holding the coil carriage to the galvanometer framework. These screws pass through large holes in the carriage so as to allow of some adjustment.
Fig. 82.
Fig. 83.
§ 104. Glass.
When glass is properly chosen and perfectly dry it has insulating properties possibly equal to those possessed by quartz or crystalline sulphur. For many purposes, however, its usefulness is seriously reduced by the persistence with which it exhibits the phenomena of residual charge, and the difficulty that is experienced in keeping it dry.
The insulating power of white flint glass is much in excess of that of soft soda glass, which is a poor insulator, and of ordinary green bottle glass. The jars of Lord Kelvin's electrometers, which insulate very well, are made of white flint glass manufactured in Glasgow, but it is found that occasionally a particular jar has to be rejected on account of its refusing to insulate, and this, if I understand aright, even when it exhibits no visible defects.
A large number of varieties of glass were tested by Dr. Hopkinson at Messrs. Chance Bros. Works, in 1875 and 1876 (Phil. Trans, 1877), and in 1887 (Proc. Roy. Soc. xli. 453), chiefly with a view to the elucidation of the laws regulating the residual charge; and incidentally some extraordinarily high insulations were noted among the flint glasses. The glass which gave the smallest residual charge was an "opal" glass; and flint glasses were found to insulate 105 times as well as soda lime glasses. The plates of Wimshurst machines are made of ordinary sheet window glass, but as the insulating property of this material appears to vary, it is generally necessary to clean, dry, and test a sheet before using it. With regard to hard Bohemian glass, this is stated by Koeller (Wien Bericht) to insulate ten times as well as the ordinary Thuringian soft soda glass.
On the whole the most satisfactory laboratory practice is to employ good white flint glass. The only point requiring attention is the preparation of the glass by cleaning and drying. Of course all grease or visible dirt must be removed as described in an earlier chapter (§ 13), but this is only a beginning. The glass after being treated as described and got into such a state as to its surface that clean water no longer tends to dry off unequally, must be subjected to a further scrub with bibulous paper and a clear solution of oleate of soda. The glass is then to be well rinsed with distilled water and allowed to drain on a sheet of filter paper.
A very common cause of failure lies in the contamination of the glass with grease from the operator's fingers. Before setting out to clean the glass the student will do well to wash his hands with soap and water, then with dilute ammonia and finally with distilled water.
In the case of an electrometer jar which has become conducting but is not perceptibly dirty, rubbing with a little oleate of soda and a silk ribbon, followed, of course, by copious washing, does very well. If there is any tin-foil on the jar, great care must be taken not to allow the glass surface to become contaminated by the shellac varnish or gum used to stick the tin-foil in position.
Finally, the glass should be dried by radiant heat and raised to a temperature of 100° C. at least, and kept at it for at least half an hour. Before drying it is of course advisable to allow the water to drain away as far as possible, and if the water is only the ordinary distilled water of the laboratory, the glass is preferably wiped with a clean bit of filter paper; any hairs which may be left upon the glass will brush off easily when the glass is dry.
In order to obtain satisfactory results the glass must be placed in dry air before it has appreciably cooled. This is easily done in the case of electrometer jars, and so long as the air remains perfectly dry through the action of sulphuric acid or phosphorus pentoxide, the jar will insulate. The slightest whiff of ordinarily damp air will, however, enormously reduce the insulating power of the glass, so that unvarnished glass surfaces must be kept for apparatus which is practically air-tight.
For outside or imperfectly protected uses the glass does better when varnished. It is a fact, however, that varnished glass is rarely if ever so good as unvarnished glass at its best. Too much care cannot be taken over the preparation of the varnish; French polish, or carelessly made shellac varnish, is likely to do more harm than good.
The best orange shellac must be dissolved in good cold alcohol by shaking the materials together in a bottle. The alcohol is made sufficiently pure by starting with rectified spirit and digesting it in a tin flask over quick-lime for several days, a reversed condenser being attached. A large excess of lime must be employed, and this leads to a considerable loss of alcohol, a misfortune which must be put up with.
After, say, thirty hours' digestion, the alcohol may be distilled off and employed to act on the shellac. In making varnish, time and trouble are saved by making a good deal at one operation--a Winchester full is a reasonable quantity. The bottle may be filled three-quarters full of the shellac flakes and then filled up with alcohol; this gives a solution of a convenient strength.
The solution, however, is by no means perfect, for the shellac contains insoluble matter, and this must be got rid off.`' One way of doing this is to filter the solution through the thick filtering paper made by Schleicher and Schuell for the purpose, but the filtering is a slow process, and hence requires to be conducted by a filter paper held in a clip (not a funnel) under a bell jar to avoid evaporation.
Another and generally more convenient way in the laboratory is to allow the muddy varnish to settle--a process requiring at least a month--and to decant the clear solution off into another bottle, where it is kept for use. The muddy residue works up with the next lot of shellac and alcohol, which may be added at once for future use.
The glass to be varnished is warmed to a temperature of, say, 50° C, and the varnish put on with a lacquering brush; a thin uniform coat is required. The glass is left to dry long enough for the shellac to get nearly hard and to allow most of the alcohol to evaporate. It is then heated before a fire, or even over a Bunsen, till the shellac softens and begins to yield its fragrant characteristic smell.
If the coating is too heavy, or if the heating is commenced before the shellac is sufficiently dry, the latter will draw up into "tears," which are unsightly and difficult to dry properly. On no account must the shellac be allowed to get overheated. If the varnish is not quite hard when cold it may be assumed to be doing more harm than good.
In varnishing glass tubes for insulating purposes it must be remembered that the inside of the tube is seldom closed perfectly as against the external air, and consequently it also requires to be varnished. This is best done by pouring in a little varnish considerably more dilute than that described, and allowing it to drain away as far as possible, after seeing that it has flooded every part of the tube.
During this part of the process the upper end of the tube must be closed, or evaporation will go on so fast that moisture will be deposited from the air upon the varnished surface. Afterwards the tube may be gently warmed and a current of air allowed to pass, so as to prevent alcohol distilling from one part of the tube to another. The tube is finally heated to the softening point of shellac, and if possible closed as far as is practicable at once.
§ 105. Ebonite or Hard Rubber.
This exceedingly useful substance can be bought of a perfectly useless quality. Much of the ebonite formerly used to cover induction coils for instance, deteriorates so rapidly when exposed to the air that it requires to have its surface renewed every few weeks.
The very best quality of ebonite obtainable should be solely employed in constructing electric works. It is possible to purchase good ebonite from the Silvertown Rubber Company (and probably from other firms), but the price is necessarily high, about four shillings per pound or over.
At ordinary temperatures ebonite is hard and brittle and breaks with a well-marked conchoidal fracture. At the temperature of boiling water the ebonite becomes somewhat softened, so that it is readily bent into any desired shape; on cooling it resumes its original hardness.
This property of softening at the temperature of boiling water is a very valuable one. The ebonite to be bent or flattened is merely boiled for half an hour or so in water, taken out, brought to the required shape as quickly as possible, and left to cool clamped in position.
The sheet ebonite as it comes from the makers is generally far from flat. It is often necessary to flatten a sheet of ebonite, and of course this is the more easily accomplished the smaller the sheet. Consequently a bit of ebonite of about the required size is first cut from the stock sheet by a hack-saw such as is generally used for metals. This piece is then boiled and pressed between two planed iron plates previously warmed to near 100° C.
With pieces of ebonite such as are used for the tops of resistance boxes, measuring, say, 20 X 8 X 11 inches, very little trouble is experienced. The sheets when cold are found to retain the flatness which has been forced upon them perfectly well. It is otherwise with large thin sheets such as are used for Holtz machines. I have succeeded fairly, but only fairly, by pressing them in a "gluing press," consisting of heavy planed iron slabs previously heated to 100° C.
I do not know exactly how best to flatten very thin and large sheets. It is easy to make large tubes out of sheet ebonite by taking advantage of the softening which ebonite undergoes in boiling water. A wooden mandrel is prepared of the proper size and shape. The ebonite is softened and bent round it; this may require two or three operations, for the ebonite gets stiff very quickly after it is taken out of the water. Finally the tube is bound round the mandrel with sufficient force to bring it to the proper shape and boiled in water, mandrel and all. The bath and its contents are allowed to cool together, so that the ebonite cools uniformly.
Tubes made in this way are of course subject to the drawback of having an unwelded seam, but they do well enough to wind wire upon if very great accuracy of form is not required. If very accurate spools are needed the mandrel is better made of iron or slate and the spool is turned up afterwards. The seam may be strapped inside or at the ends by bits of ebonite acting as bridges, and the seam itself may be caulked with melted paraffin or anthracene.
Working Ebonite.
Ebonite is best worked as if it were brass, with ordinary brass-turning or planing tools. These tools should be as hard as possible, for the edges are apt to suffer severely, and blunt tools leave a very undesirable woolly surface on the ebonite. In turning or shaping ebonite sheets it is as well to begin by taking the skin off one side first, and then reversing the sheet, finishing the second side, and then returning to the first. This is on account of the fact that ebonite sometimes springs a little out of shape when the skin is removed.
Turned work in ebonite, if well done, requires no sand-papering, but may be sufficiently polished by a handful of its own shavings and a little vaseline. The advantage of using a polished ebonite surface is that such a surface deteriorates more slowly under the influence of light and air than a surface left rough from the tool. If very highly polished surfaces are required, the ebonite after tooling is worked with fine pumice dust and water, applied on felt, or where possible by means of a felt buff on the lathe, and finally polished with rouge and water, applied on felt or cloth.
Ebonite works particularly well under a spiral milling cutter, and sufficiently well under an ordinary rounded planing tool, with cutting angle the same as for brass, and hardened to the lightest straw colour.
It is not possible, on the other hand, to use the carpenter's plane with success, for the angle of the tool is too acute and causes the ebonite to chip.
In boring ebonite the drill should be withdrawn from the hole pretty often and well lubricated, for if the borings jam, as they are apt to do, the heat developed is very great and the temper of the drill gets spoiled. Ebonite will spoil a drill by heating as quickly as anything known; on the other hand, it may be drilled very fast if proper precaution is taken.
It is advisable to expose ebonite to the light as little as possible, especially if the surface is unpolished, for under the combined action of light and air the sulphur at the surface of the ebonite rapidly oxidises, and the ebonite becomes covered with a thin but highly conducting layer of sulphurous or sulphuric acid or their compounds. When this happens the ebonite may be improved by scrubbing with hot water, or washing freely with alcohol rubbed on with cotton waste in the case of apparatus that cannot be dismounted.
A complete cure, however, can only be effected by scraping off the outer layer of ebonite so as to expose a fresh surface. For this purpose a bit of sheet glass broken so as to leave a curved edge is very useful, and the ebonite is then scraped like a cricket bat. In designing apparatus for laboratory use it is as well to bear in mind that sooner or later the ebonite parts will require to be taken down and scraped up. Rods or tubes are, of course, most quickly treated on the lathe with rough glass cloth, and may be finished with fine sandpaper, then pumice dust and water, applied on felt. After cleaning the pumice off by means of water and a rag, the final touch may be given by means of vaseline, applied on cloth or on ebonite shavings.
§ 106. Mica.
A great variety of minerals go under this name. Speaking generally, the Russian micas coming into commerce are potash micas, and mica purchased in England may be taken to be potash mica, especially if it is in large sheets.
At ordinary temperatures "mica" of the kind found in commerce is an excellent insulator. Schultze (Wied. Ann. vol. xxxvi. p. 655) comes to the conclusion that both at high and at low temperatures mica (of all kinds?) is a better insulator than white "mirror glass," the composition of which is not stated. The experiments of the author referred to were apparently left unfinished, and altogether too much stress must not be laid on the results obtained, one of which was that mica conducts electrolytically to some extent at high temperatures.
Bouty (Journal de Physique, 1890 [9], 288) and J. Curie (Thèse de Doctorat, Paris, 1888) agree in making the final conductivity of the mica used in Carpentier's condensers exceedingly small--at all events at ordinary temperatures. Bearing in mind that for such substances the term specific resistance has no very definite meaning, M. Bouty considers it is not less than 3.19 x 1028 E.M. units at ordinary temperatures. M. Bouty gives a note or illustration of what such numbers mean--a precaution not superfluous in cases where magnitudes are denoted logarithmically. Referring to the value quoted, viz. 3.19 x 1028, M. Bouty says, "Ce serait la resistance d'une colonne de mercure de 1mmq de section et de longueur telle que la lumière se propageant dans le vide, mettrait plus de 3000 ans A se transmettre d'une extrémité à I'autre de la colonne."
M. Bouty returns to the study of mica (muscovite) in the Journal de Physique for 1892, p. 5, and there deals with the specific inductive capacity, which for a very small period of charge he finds has the value 8--an enormous value for such a good insulator, and one that it would be desirable to verify by some totally distinct method. This remark is enforced by the fact that M. Klemencic finds the number 6 for the same constant. The temperature coefficient of this constant was too small for M. Bouty to determine. The electric intensity was of the order of 100 volts per centimetre, and the experiments seem to indicate that the specific inductive capacity would be only slightly less if referred to a period of charge indefinitely short.
I have found that the residual charge in a mica condenser, made according to Carpentier's method (to be described below), is about 1 per cent of the original charge under the following circumstances.
Voltage 300 volts on a plate 0.2 mm. thick, duration of charge ten minutes, temperature about 20° C. To get this result the mica must be most carefully dried. This and other facts indicate that the so-called residual charge on ordinary condensers is, to a very large extent, due to the creeping of the charge from the armatures over the more or less conducting varnished surfaces of the mica, and its slow return on discharge.
This source of residual charge was carefully guarded against by Rowland and Nichols (Phil. Mag. 1881) in their work on quartz, and is referred to by M. Bouty, who adduces some experiments to show that his own results are not vitiated by it. On the other hand, M. Bouty shows that a small rise in temperature enormously affects the state of a mica surface, and that the surface gets changed in such a way as to become very fairly conducting at 300° C. Also anybody can easily try for himself whether exposing a mica condenser plate which has been examined in presence of phosphorus pentoxide to ordinary air for five minutes will not enormously increase the residual charge, as has always been the case in the writer's experience, and if so, it is open to him to suggest some cause other than surface creeping as an explanation.
M. Bouty, using less perfectly dried mica, did not get so good a result as to smallness of residual charge as the one above quoted.
The chief use of mica for laboratory purposes depends on the ease with which it can be split, and also upon the fact that it may be considerably crumpled and bent without breaking. It therefore makes an excellent dielectric in so far as convenience of construction is concerned in the preparation of condensers, and lends itself freely to the construction of insulating washers or separators of any kind. Its success as a fair insulator at moderate temperatures has led to its use in resistance thermometers, where it appears to have given satisfaction up to, at all events, 400° C.
It is worth a note that according to Werner Siemens, who had immense experience (Wied. Ann. vol. clix.), soapstone is the only reliable insulator at a red heat, but, no doubt, a good deal depends on the particular specimen investigated.
§ 107. Use of Mica in Condensers.
If good results are desired it is essential to select the mica very carefully. Pieces appreciably stained,--particularly if the stain is not uniformly distributed,--cracked pieces, and pieces tending to flake off in patches should be rejected. The best samples of mica that have come under the writer's observation are those sheets sold for the purpose of giving to silver photographic prints that hideous glazed surface which some years ago was so popular.
Sheets of mica about 0.1 to 0.2 mm. thick form good serviceable condenser plates, and will certainly stand a pressure of 300 volts, and most likely a good deal more. The general practice in England seems to have been to build up condensers of alternate sheets of varnished or paraffined-mica and tin-foil.
This practice is open to several objections. In the first place, the capacity of a condenser made in this way varies with the pressure binding the plates together. In the second place, the amount of mica and tin-foil required is often excessive in consequence of the imperfect contact of these substances. Again, the inevitable air film between the mica and tin-foil renders condensers so made unsuitable for use with alternating currents, owing to the heating set up through air discharges, and which is generally, though often (if not always) wrongly, attributed to dielectric hysteresis.
These imperfections are to a great extent got over by M. Carpentier's method of construction, which is, however, rather more costly both in material and labour. On the other hand, wonderful capacities are obtained with quite small amounts of mica. M. Bouty mentions a condenser of one microfarad capacity weighing 1500 grms. and contained in a square box measuring 12 centimetres on the side, and about 3 centimetres thick.
The relation between the capacity and surface of doubly-coated plates is in electro-static units:
Capacity = (sp. ind. capacity X area of one surface)/(4pi X thickness)
This may be reduced to electro-magnetic units by dividing by 9x10^20, and to microfarads by further multiplying by 10^15.
M. Carpentier begins, of course, by having his mica scrupulously clean and well selected. It is then silvered by one of the silvering processes (§ 65) on both sides, for which purpose the sheets may be suspended in a paraffined wood rack, so as to lie horizontally in the silvering solution, a space of about half an inch being allowed between the sheets. The silvering being finished, the sheets are dipped along two parallel edges in 75 per cent nitric acid. With regard to the third and fourth edges of the sheet, the silver is removed on one side only, using a spun glass brush; if we agree to call the two surfaces of the mica A and B respectively, and the two edges in question C and D, then the silver is removed from the A side along edge C, and from the B side along edge D. The silvered part is shown shaded in Fig. 84. By this arrangement the silver and mica plates may be built up together so as to form the same mutual arrangement of contacts as in an ordinary mica tin-foil condenser.
Fig. 84.
It need hardly be said that the sheets require very complete washing after treatment with nitric acid, followed by a varnishing of the edges as already described in the case of glass, and baking at a temperature of 140° C. in air free from flame gases, till the shellac begins to emit its characteristic odour and is absolutely hard when cold.
The plates are then built up so as to connect the sheets which require to be connected, and to insulate the other set. General contact is, if necessary, secured by means of a little silver leaf looped across from plate to plate--a part of the construction which requires particular attention and clean hands, for it is by no means so easy to make an unimpeachable contact as might at first appear.
The condenser, having been built up, may be clamped solid and placed in its case; the capacity will not depend appreciably on the tightness of the clamp screws--a great feature of the construction. Such a condenser will not give its best results unless absolutely dry. I have kept one very conveniently in a vacuum desiccator over phosphorus pentoxide, but if of any size, the condenser deserves a box to itself, and this must be air-tight and provided with a drying reagent, so arranged that it can be removed through a manhole of some sort.
Contact to the brass-work on the lid may be made by pressing spring contacts tightly down upon the ends of the silver foils and carrying the connections through the lid. This also serves to secure the condenser in position.
§ 108. Micanite.
This substance, though probably comparing somewhat unfavourably with the insulators already enumerated, and being subject to the uncertainties of manufacture, has during the last few years achieved a considerable success in American electrical engineering construction. It is composed of scrap mica and shellac varnish worked under pressure to the desired shape, and may be obtained in sheets, plates, and rods, or in any of the forms for which a die happens to have been constructed.
Of course, in special cases it would be worth while to prepare a die, and then the attainable forms would be limited by moulding considerations only. The writer's experience is very limited in this matter, but Dr. Kennelly, with whom he communicated on the subject, was good enough to reply in favour of micanite for engineering work.
§ 109. Celluloid.
This material is composed of nitrocellulose and camphor.
It has fair insulating properties, and may be obtained in a variety of forms, but has now been generally abandoned for electrical work on account of its inflammability.
§ 110. Paper.
Pure white filter paper, perfectly dry, is probably a very fair insulator; the misfortune is that in practice it cannot be kept dry. Under the most favourable circumstances its specific resistance may approach 1024 E.M. units. It must therefore be considered rather as a partial conductor than as an insulator. The only case of the use of dry paper as an insulator in machine construction which has come under the writer's notice is in building up the commutators of the small motors which used to drive the Edison phonographs.
Its advantages in this connection are to be traced to the fact that a commutator so built up is durable and keeps a clean surface. Of course, the use of paper as an insulator for telephone wires is well known, but its success in this direction depends less upon its insulating properties than upon the fact that it can be arranged in such a way as to allow of the wires being partially air insulated, an arrangement tending to reduce the electrostatic capacity of the wire system.
At one time it was the custom of instrument makers to employ ordinary printed paper in the shape of leaves torn from books or the folios of old ledgers to form the dielectric of the condensers used in connection with the contact breakers of induction coils. This practice has nothing but economy to recommend it, for cases often occur in which the paper, by gradual absorption of moisture from the air, comes to insulate so badly that it practically short circuits the spark gap, and so stops the action of the coil. Three separate cases have come within the writer's experience.
Some measurements of the resistance of paper have been made by F. Uppenborn (Centralblatt fuer Electrotechnik, Vol. xi. p. 215, 1889). There is an abstract of the paper also in Wiedemann's Beiblaetter (1889, vol. xiii. P. 711). Uppenborn examined the samples of paper under normal conditions as to moisture and obtained the following results:-
Description of Paper
I
Pressure Intensity
II.
Specific Resistance corresponding to pressures as in Column I. Ohms.
III
Pressure Intensity.
IV.
Specific Resistance corresponding to Column III. Ohms.
Common cardboard 2.3 mm. thick
0.05 kilo. per 6000 sq. mm.
4.85 x 1015
20 kg. per 6000 sq. mm.
4.7 x 1014
Gray paper, 0.26 mm. thick
0.05 kilo. per 5000 sq. mm.
3.1 x 10^15
20 kg. per 5000 sq. mm.
8 x 1014
Yellow parchment paper-09 mm. thick
0.05 kilo. per 5300 sq. mm.
3.05 x 1016
20 kg. per 5300 sq. mm.
8 x 1016
Linen tracing cloth
0.05 kilo. per 6000 sq. mm.
1.35 x 1016
20 kg. per 33,000 sq. mm.
1.86 x 10^15
§ 111. Paraffined Paper.
Like wood and other semiconductors, paper can be vastly improved as an insulator by saturating it with melted paraffin. To get the best results a pure paper free from size must be employed--gray Swedish filter paper does well. This is dried at a temperature above 100° C. for, say, half an hour, and the sheets are then floated on the top of paraffin, kept melted at 140° C. or thereabout in a baking dish. As soon as the paper is placed upon the melted paraffin the latter begins to soak through, in virtue of capillary action, and drives before it the air and moisture, causing a strongly marked effervescence.
After about one minute the paper may be thrust below the paraffin to soak. When a sufficient number of papers have accumulated, and when no more gas comes off, the tray may be placed in a vacuum box (Fig. 85), and the pressure reduced by the filter pump. As the removal of the air takes time, provision must be made for keeping the bath hot.
A vacuum may be maintained for about an hour, and air then readmitted. Repeated exhaustions and readmissions of air, which appear to improve wood, do not give anything like such a good result with paper. In using a vacuum box provision must be made in the shape of a cool bottle between the air pump and the box. If this precaution be omitted, and if any paraffin splashes on to the hot surface of the box, it volatilises with decomposition and the products go to stop up the pump. Paraffin with a melting-point of 50° C. or upwards does well.
The bath should be allowed to cool to about 60° C. before the papers are removed, so that enough paraffin may be carried out to thoroughly coat the paper and prevent the entrance of air.
Fig. 85.
Fig. 85 is a section of a vacuum vessel which has been found very convenient. It measures about two feet in diameter at the top. It is round, because it is much easier to turn one circular surface than to plane up four surfaces, which has to be done if the box is square. Both the rim of the vessel and the approximating part of the cover require to be truly turned and smoothly finished. A very good packing is made of solid indiarubber core about half an inch thick. This is carefully spliced--cemented by means of a solution of rubber in naphtha, and the splice sewed by thick thread. The lid ought to have a rim fitting inside the vessel, for this keeps the rubber packing in place; the rim has been accidentally omitted in Fig. 85. The bolts should not be more than five inches apart, and should lie at least half an inch in diameter, and the rim and lid should be each half an inch thick.
Condensers may now be built up of sheets of this prepared paper interleaved with tin-foil in the ordinary way. If good results are required, the condenser when finished is compressed between wooden or glass end-pieces by means of suitable clamps. It can then be put in a box of melted paraffin, heated up to 140° C, and exhausted by means of the water pump for several hours.
In this process the air rushes out from between the paper and foils with such vehemence that the paraffin is generally thrown entirely out of the box. To guard against this the box must be provided with a loosely fitting and temporary lid, pierced with several holes.
The real test as to when exhaustion is complete would be the cessation of any yield of air or water. Since it is not generally convenient to make the vacuum box so air-tight that there are absolutely no leaks at all, and as the paraffin itself is, I think, inclined to "crack" slightly at the temperature of 140° C, this test or criterion cannot be conveniently applied.
Two exhaustions, each of about two hours' duration, have, however, in my experience succeeded very well, provided, of course, that the dielectric is prepared as suggested. At the end of the exhaustion process the clamping screws are tightened as far as possible, the condenser remaining in its bath until the paraffin is pasty.
Condensers made in this way resist the application of alternating currents perfectly, as the following tests will show. The dielectric consisted of about equal parts of hard paraffin and vaseline. A condenser of about 0.123 microfarads capacity and an insulation resistance of 2000 megohms, [Footnote: As tested by a small voltage.] having a tin-foil area of 4.23 square metres (about), and double papers each about 0.2 mm. thick, designed to run at 2000 volts with a frequency of 63 complete periods, was tested at this frequency.
The condenser was thoroughly packed all round in cotton-wool to a thickness of 6 inches, and its temperature was indicated more or less by a thermometer plunged through a hole in the lid of the containing box and of the condenser box, and resting on the upper surface of one set of tin-foil electrodes, from which the soft paraffin mixture had been purposely scraped away. The following were the results of a four hours' run at a voltage 50 per cent higher than that for which the condenser was designed--i.e. 3000 volts.
Times. Voltage Temperature Temperature Difference in Condenser. in Air.
Hrs. Min.
2 10 2750 22.8° C. 23.0° C. + 0.2°
3 10 2700 23.0° C. 23.3° C. + 0.3°
3 18 3200 23.1° C. 23.0° C. -0.1°
4 10 3200 23.3° C. 23.7° C. + 0.4°
5 10 3100 23.6° C. 23.4° C. -0.2°
6 10 3000 23.8° C. 23.35° C. -0.45°
An idea of the order of the amount of waste may be formed from the following additional experiment.
A condenser similar to the one described was filled with oil of a low insulating power. It was tested calorimetrically, and also by the three voltmeter method, which, however, proved to be too insensitive. The temperature rise in the non-conducting box in air was about 0.3° C. per hour, and the loss of power was found to be less than 0.1 per cent. In the present case the actual rise was only 1° in four hours, and the integral give and take between the condenser and the air is practically nothing; consequently we may consider with safety that the rate of rise is certainly less than 1 degree per three hours. The voltage and frequency were about the same in both experiments, consequently the energy passed is about proportional to the capacity used in the two experiments.
From this it follows that since the specific heat of both condensers was the same (nearly), the loss in the present case is a good deal less than one-tenth per cent. The residual charge is also much less than when the condenser is simply built up of paper paraffined in an unsystematic manner, and from which the air and water have been imperfectly extracted, as by baking the condenser first, and then immersing it in paraffin or oil.
It is usual to consider that the phenomena of residual charge and heating in condensers, to which alternating voltages are applied, are closely allied. This is true, but the alliance is not one between cause and effect--at all events, with regard to the greater part of the heating. The imperfect exclusion of air and moisture, particularly the latter, certainly increases the residual charge by allowing surface creeping to occur; but it also acts directly in producing heating, both by lowering the insulation of the condenser and by allowing of air discharges between the condenser plates.
Of these causes of heating, the discharges in air or water vapour are probably the more important. Long ago a theory of residual charge was given by Maxwell, based on the consideration of a laminated dielectric, the inductivity and resistance of which varied from layer to layer. It was shown that such an arrangement, and hence generally any want of homogeneity in a direction inclined to the lines of force leading to a change of value of the product of specific resistance and specific inductive capacity, would account for residual charge.
This possible explanation has been generally accepted as the actual explanation, and many cases of residual charge attributed to want of homogeneity, which are certainly to be explained in a simpler manner. For instance, the residual charge in a silvered mica plate condenser, carefully dried, can be increased at least tenfold by an exposure of a few minutes to ordinarily damp air. The same result occurs with condensers of paraffined or sulphured paper; and these are the residual changes generally observed. The greater part must be due to creeping.
§ 112. Paraffin.
This substance has long enjoyed great popularity in the physical laboratory. Its specific resistance is given by Ayrton and Perry as more than 1025, but it is probably much higher in selected samples. The most serviceable kind of paraffin is the hardest obtainable, melting at a temperature of not less than 52° C. It is a good plan to remelt the commercial paraffin and keep it at a temperature of, say, 120° C. for an hour, stirring it carefully with a glass rod so that it does not get overheated; this helps to get rid of traces of water vapour.
Hard paraffin, when melted, has an enormous rate of expansion with temperature, so great, indeed, that care must be taken not to overfill the vessels in which it is to be heated. Castings can only be prepared by cooling the mould slowly from the bottom, keeping the rest of the mould warm, and adding-paraffin from time to time to make up for the contraction. The cooling is gradually allowed to spread up to the free surface.
The chief use of paraffin in the laboratory is in the construction of complicated connection boards, which are easily made by drilling holes in a slab of paraffin, half filling them with mercury, and using them as mercury cups.
Since paraffin is a great collector of dust, it should be screened by paper, otherwise the blocks require to be scraped at frequent intervals, which, of course, electrifies them considerably. This electrification is often difficult to remove without injuring the insulating power of the paraffin. A light touch with a clean Bunsen flame is the readiest method, and does not appear to reduce the insulation so much as might be expected. The safest way, however, is to leave the key covered by a clean cloth, which, however, must not touch the surface, for a sufficient time to allow of the charges getting away.
The paraffin often becomes electrified itself by the friction of the key contacts, so that in electrometer work it is often convenient to form the cups by lining them with a little roll of copper foil twisted up at the bottom. In this case the connecting wires should, of course, be copper. Small steel staples are convenient for fastening the collecting wires upon the paraffin; or, in the case where these wires have to be often removed and changed about, drawing-pins are very handy.
With mercury cups simply bored in paraffin great trouble will often be experienced in electrometer work, owing to a potential difference appearing between the cups--at all events when the contacts are inserted and however carefully this be done. A few drops of very pure alcohol poured in above the mercury often cures this defect. The surface of paraffin is by no means exempt from the defect of losing its insulating power when exposed to damp air, but it is not so sensitive as glass, nor does the insulating power fall so far.
Two useful appliances are figured.
Fig. 86. Fig. 87.
One, in which paraffin appears as a cement, is an insulating stand made out of a bit of glass or ebonite tube cemented into an Erlenmeyer flask, having its neck protected from dust when out of use by a rubber washer, the other a "petticoat" insulator made by cementing a flint glass bottle into a glass dish with paraffin. In course of time the paraffin will be found to have separated from the glass. When this occurs the apparatus may be melted together again by placing it on the water bath for a few minutes.
§ 113. Vaseline, Vaseline Oil, and Kerosene Oil.
These, when dry, insulate almost, but not quite as well as solid paraffin. H. Koeller (Wien Berichte, 98, ii. 201, 1889; Beibl. Wied. Ann. 1890, p. 186), working with very small voltages, places the final(?) specific resistance of commercial petroleum, ether, and vaseline oil at about 2 X 1027 C.G.S. This is ten times higher than the value assigned to commercial benzene (C6H6), and a hundred times higher than the value for commercial toluene.
All these numbers mean little or nothing, but the petroleum and vaseline oil were the best fluid insulators examined by Koeller. By mixing vaseline with paraffin a soft wax may be made of any desired degree of softness, and by dissolving vaseline in kerosene an insulating liquid of any degree of viscidity may be obtained.
Hard paraffin may be softened somewhat by the addition of kerosene, and an apparently homogeneous mass cast from the mixture. It will be found, however, that in course of time the kerosene oozes out, unless present in very small quantity. Koeller has found (loc. cit.) that some samples of vaseline oil conducted "vollstaendig gut," but I have not come across such samples. Vaseline oil, however, is sold at a price much above its value for insulating purposes.
Kerosene oil is best obtained dry by drawing it directly from a new tin and exposing it to air as little as possible. Of course, it may be dried by chemical means and distillation, but this is usually (or always) unnecessary.
Fig 88.
There is some danger of kerosene containing minute traces of sulphuric acid, and it and other oils may be conveniently tested for insulation in the following manner. The quartz electroscope is taken, and the insulating rod heated in the blow-pipe. The electroscope will now insulate well enough to show no appreciable collapse of the leaves in one or two hours' time. Upon the plate of the electroscope is put a platinum or copper cylinder, and this is filled with kerosene (say) up to a fixed mark.
The electroscope is placed on a surface plate, or, at all events, on a sheet of plate glass, and a "scribing block" is placed along side it and the scriber adjusted to dip into the kerosene to any required depth. This is done by twisting a bit of wire round the scribing point and allowing it to project downwards. The point itself serves to give an idea of the height to which the vessel may be filled. The liquid is adjusted till its surface is in contact with the end of the scribing point, and the wire then projects into the liquid and forms an electrode of constant area of surface. The scribing block is put to earth. A charge is given to the electroscope, and the time required for a given degree of collapse of the leaves noted.
The kerosene is then removed and its place taken by vaseline or paraffin, known to insulate well as a standard for comparison. The experiment is then repeated, and the time noted for the same degree of collapse. This test, though of course rough, is generally quite sufficient for workshop purposes, and is easily applied. Moreover, it does not require correction for electrometer leakage, as generally happens when more elaborate appliances are used.
The actual resistance of insulating oils depends so much on the electrical intensity, on the duration of that intensity, and on the previous history of the oil as to the direction of the voltage to which it has been subjected--to say nothing of the effects of traces of moisture--that quantitative experiments are of no value unless they are extremely elaborate. I shall therefore only append the following numbers due to Bouty, Ann. de Chemie et de Physique (6), vol. xxvii. p. 62, 1892, in which the effect of the conductivity on the determination of the specific inductive capacity was properly allowed for:-
Carbon
Bisulphide.
Turpentine.
Benzene (C6H6) at 20° C.
Benzene at
60° C.
Specific inductive capacity
2.715
2.314
2.21
2.22
Specific resistance in ohms per cubic centimetre
1.5 x 1013,
1.75 x 1012
1.56 x 1011
7.9 x 1011
[Footnote: Professor J. J. Thomson, and Newall (Phil. Proc. 1886) consider that carbon bisulphide showed traces of a "residual charge" effect; hence, until this point is cleared up, we must regard Bouty's value as corresponding only to a very short, but not indefinitely short, period of charge. On this point the paper must be consulted.
March 1897--The writer has investigated this point by an independent method, but found no traces of "residual charge."]
Information as to the specific inductive capacity of a large number of oils may be found in a paper by Hopkinson, Phil. Proc. 1887, and in a paper by Quincke in Wiedemann's Annalen, 1883.
§ 114. Imperfect Conductors.
Under this heading may be grouped such things as wood, slate, marble, etc--in fact, materials generally used for switchboard insulation. An examination of the insulating power of these substances has recently been made by B. O. Peirce (Electrical Review, 11th January 1895) with quite sufficient accuracy, having in view the impossibility of being certain beforehand as to the character of any particular sample. The tests were made by means of holes drilled in slabs of the material to be examined. These holes were three-eighths of an inch in diameter, and from five-eighths to three-quarters of an inch deep, and one inch apart, centre to centre. A voltage of about 15 volts was employed. The following general results were arrived at:-
(1) Heating in a paraffin bath always increases the resistance of wood, though only slightly if the wood be hard and dense.
(2) Frequent exhaustion and readmission of air above the surface of the paraffin always has a good effect in increasing the resistance of wood.
(3) When wood is once dry, impregnating it with paraffin tends to keep it dry.
(4) Red vulcanised fibre, like wood, absorbs paraffin, but it cannot be entirely waterproofed in this way.
(5) The resistance of wood with stream lines along the grain is twenty to fifty per cent lower than when the stream lines cross the grain.
(6) The "contact" resistance between slabs of wood pressed together is always very high.
The following table will sufficiently illustrate the results obtained. The stone was dried in the sun for three weeks in the summer (United States), and the wood is described as having been well seasoned:-
CURRENT WITH THE GRAIN
Lowest Resistance Highest Resistance Lowest Specific Highest Specific between two Cups between two Cups Resistance in Resistance in in Megohms. in Megohms. Megohms. Megohms.
Ash.
550 920 380 700
Cherry
1100 4000 2800 6000
Mahogany
430 730 310 610
Oak
220 420 1050 2200
Pine.
330 630 360 1470
Hard pine.
10 48 17 1050
Black walnut
1100 3000 320 2100
Red fibre
2 4 3 60
Slate
184 280
Soapstone.
330 500
White marble
2000 8800
§ 115. As to working the materials very little need be said.
Fibre is worked like wood, but has the disadvantage of rapidly taking the edge off the tools. In turning it, therefore, brass-turning tools, though leaving not quite such a perfect finish as wood-turning tools, last much longer, and really do well enough. Fibre will not bear heating much above 100°C--at all events in paraffin. At 140° C. it becomes perfectly brittle. Its chief merit lies in its great strength. So far as insulation is concerned, Mr. Peirce's experiments show that it is far below most kinds of wood.
Slate. This is a vastly more useful substance than it is generally credited with being. It is very easily worked at a slow speed, either on the shaping machine or on the lathe, with tools adjusted for cutting brass, and it keeps its figure, which is more than can be said for most materials. It forms a splendid base for instruments, especially when ground with a little emery by iron or glass grinders, fined with its own dust, and French polished in the ordinary way. Spools for coils of considerable radial dimension may be most conveniently made of slate instead of wood or brass, both because it keeps its shape, and because it insulates sufficiently well to stop eddy currents--at all events, sufficiently for ordinary practice. An appreciable advantage is that slate may be purchased at a reasonable rate in large slabs of any desired thickness. It is generally cut in the laboratory by means of an old cross-cut saw, but it does not do much damage to a hard hack saw such as is used for iron or brass.
Marble. According to Holtzapffell, marble may be easily turned by means of simple pointed tools of good steel tempered to a straw colour. The cutting point is ground on both edges like a wood-turning tool, and held up to the work "at an angle of twenty or thirty degrees" (?with the horizontal). The marble is cut wet to save the tool. As soon as the point gets, by grinding, to be about one-eighth of an inch broad it must either be drawn down or reground; a flat tool will not turn marble at all.
A convenient saw for marble is easily made on the principle of the frame saw. A bit of hoop iron forms a convenient blade, and is sharpened by being hammered into notches along one edge, using the sharp end of a hammer head. The saw is liberally supplied with sand and water--or emery and water, where economy of time is an object. The sawing of marble is thus really a grinding process, but it goes on rapidly. Marble is ground very easily with sand and water; it is fined with emery and polished with putty powder, which, I understand, is best used with water on cloth or felt. As grinding processes have already been fully described, there is no need to go into them here. I have no personal knowledge of polishing marble.
§ 116. Conductors.
The properties of conductors, more particularly of metals, have been so frequently examined, that the literature of the subject is appallingly heavy. In what follows I have endeavoured to keep clear of what might properly appear in a treatise on electricity on the one hand, and in a wiring table on the other. The most important work on the subject of the experimental resistance properties of metals has been done by Matthieson, Phil. Trans. 1860 and 1862, and British Association Reports (1864); Callender, Phil. Trans. vol. clxxiii; Callender and Griffiths, Phil. Trans. vol. clxxxii; The Committee of the British Association on Electrical Standards from 1862 to Present Time; Dewar and Fleming, Phil. Mag. vol. xxxvi. (1893);
Klemencic, Wiener Sitzungsberichte (Denkschrift), 1888, vol. xcvii. p. 838; Feussner and St. Lindeck, Zeitsch. fuer Inst. 'Kunde, ix. 1889, p. 233, and B. A. Reports, 1892, p. 139. Of these, Matthieson, and Dewar and Fleming treat of resistance generally, the latter particularly at low temperatures.
[Footnote: The following is a list of Dr. Matthieson's chief papers on the subject of the electrical resistance of metals and alloys: Phil. Mag. xvi. 1858, pp. 219-223; Phil. Trans. 1858, pp. 383-388 Phil. Trans. 1860, pp. 161-176; Phil. Trans. 1862, pp. 1-27 Phil. Mag. xxi. (1861), pp. 107-115; Phil. Mag. xxiii. (1862), pp. 171-179; Electrician, iv. 1863, pp. 285-296; British Association Reports, 1863, p. 351.]
Matthieson, and Matthieson and Hockin, Klemencic, Feussner, and St. Lindeck deal with the choice of metals for resistance standards. Callender's, and Callender and Griffiths' work is devoted to the study of platinum for thermometric purposes.
The bibliography referring to special points will be given later. The simplest way of exhibiting the relative resistances of metals is by means of a diagram published by Dewar and Fleming (loc. cit.), which is reproduced on a suitable scale on the opposite page. For very accurate work, in which corrections for the volumes occupied by the metals at different temperatures are of importance, the reader is referred to the discussion in the original paper, which will be found most pleasant reading. From this diagram both the specific resistance and the temperature coefficient may be deduced with sufficient accuracy for workshop purposes. In interpreting the diagram the following notes will be of assistance. The diagram is drawn to a scale of so-called "platinum temperatures"--that is to say, let R0, R100, Rt be the resistances of pure platinum at 0°, 100°, and t° C. respectively, then the platinum temperature pt is defined as
pt = 100 X (Rt-R0)/(R100-R0)
This amounts to making the temperature scale such that the temperature at any point is proportional to the resistance of platinum at that point. Consequently on a resistance temperature diagram the straight line showing the relation between platinum resistance and platinum temperature will "run out" at the platinum absolute zero, which coincides more or less with the thermodynamic absolute zero, and also with the "perfect gas" absolute zero. Platinum temperatures may be taken for workshop purposes over ordinary ranges as almost coinciding with air thermometer temperatures. The metals used by Professors Dewar and Fleming were, with some exceptions, not absolutely pure, but in general represent the best that can be got by the most refined process of practical metallurgy. We may note further that the specific resistance is only correct for a temperature of about 15° C, since no correction for the expansion or contraction of material has been applied.
The following notes on alloys suitable for resistance coils will probably be found sufficient.
§ 117. Platinoid.
This substance, discovered by Martino and described by Bottomley (Phil. Proc. Roy. Soc. 1885), is an alloy of nickel, zinc, copper, and 1 per cent to 2 per cent of tungsten, but I have not been able to obtain an analysis of its exact composition. It appears to be difficult to get the tungsten to alloy, and it has to be added to part of the copper as phosphide of tungsten, in considerably greater quantity than is finally required. The nickel is added to part of the copper and the phosphide of tungsten, then the zinc, and then the rest of the copper. The alloy requires to be remelted several times, and a good deal of tungsten is lost by oxidation.
The alloy is of a fine white colour, and is very little affected by air--in fact, it is to some extent untarnishable. The specific resistance will be seen to be about one and a half times greater than that of German silver, and the temperature coefficient is about 0.021 per cent per degree C. (i.e. about nineteen times less than copper, and half that of German silver). To all intents and purposes it may be regarded as German silver with 1 per cent to 2 per cent of tungsten. It does not appear to have been particularly examined for secular changes of resistance.
118. German Silver. This material has been exhaustively examined of late years by Klemencic and by Feussner and St. Lindeck. Everybody agrees that German silver, as ordinarily used for resistances, and composed of copper four parts, zinc two parts, nickel one part, is very ill-fitted for the purpose of making resistance standards. This is due (1) to its experiencing a considerable increase in resistance on winding. Feussner and St. Lindeck found an increase of 1 per cent when German silver was wound on a core of ten wire diameters.
(2) To the fact that the change goes on, though with gradually decreasing rate, for months or years;
(3) to the fact that the resistance is permanently changed (increased) by heating to 40° C. or over. By "artificially ageing" coils of German silver by heating to 150° C, say for five or six hours, its permanency is greatly improved, and it becomes fit for ordinary resistance coils where changes of, say, 1/5000 do not matter.
It is a remarkable property of all nickel alloys containing zinc that their specific resistance is permanently increased by heating, whereas alloys which do not contain zinc suffer a change in the opposite direction. The manufacturers of German silver appear to take very little care as to the uniformity of the product put on the market; some so-called German silver is distinctly yellow, while other samples are bright and white.
It is noted by Price (Measurements of Electrical Resistance, p. 24) that German silver wire is apt to exhibit great differences of resistance within quite short lengths. This has been my own experience as well, and is a great drawback to the use of German silver in the laboratory, for it makes it useless to measure off definite lengths of wire with a view to obtaining an approximate resistance. In England German silver coils are generally soaked in melted hard paraffin. In Germany, at all events at the Charlottenburg Institute, according to St. Lindeck--coils are shellac-varnished and baked. In any case it appears to be essential to thoroughly protect the metal against atmospheric influence.
§ 119. Platinum Silver.
In the opinion of Matthieson and of Klemencic the 10 per cent silver, 90 per cent platinum alloy is the one most suitable for resistance standards. At all events, it has stood the test of time, for, with the following exceptions, all the British Association coils constructed of it from 1867 to the present day have continued to agree well together. The exceptions were three one-ohm coils, which permanently increased between 1888 and 1890, probably through some straining when immersed in ice. One coil changed by 0.0006 in 1 between the years 1867 and 1891. According to Klemencic, absolute permanency is not to be expected even from this alloy.
Its recommendation as a standard depends on its chemical inertness, its small temperature coefficient (0.00027 per degree), and its small thermo-voltage against copper, as the following table (taken from Klemencic) will show:-
Thermo-voltages in Micro-volts per degree against Copper over the Range 0° to 17° C.
Platinum iridium 7.14 micro-volts per degree C.
Platinum silver 6.62 micro-volts per degree C.
Nickelin 28.5 micro-volts per degree C.
German silver 10.43 micro-volts per degree C.
Manganin (St. Lindeck) 1.5 micro-volts per degree C.
Mechanically, the platinum silver is weak, and is greatly affected as to its resistance by mechanical strains--in fact, Klemencic considers it the worst substance he examined from this point of view--a conclusion rather borne out by Mr. Glazebrook's experience with the British Association standards already referred to (B. A. Reports, 1891 and 1892).
Taking everything into account, it will probably be well to construct standards either with oil insulation only, or to bake the coils in shellac before testing, instead of soaking in paraffin. Fig. 89 illustrates a form of an oil immersed standard which is in use in my laboratory, and through which a considerable current may be passed. The oil is stirred by means of a screw propeller.
Fig. 89.
Fig. 89 represents a standard resistance for making Clerk cell comparisons by the silver voltameter method. The framework on which the coils are wound consists of a base and top of slate. The pillars are of flint glass tube surrounding brass bolts, and cemented to the latter by raw shellac. Grooves are cut in the glass sleeves to hold the wires well apart. These grooves were cut by means of a file working with kerosene lubrication. A screw stirrer is provided, and the whole apparatus is immersed in kerosene in the glass box of a storage cell. The apparatus is aged to begin with by heating to a temperature a good deal higher than any temperature it is expected to reach in actual work. After this the rigidity of the frame is intended to prevent any further straining of the wire. The apparatus as figured is not intended to be cooled to 0° C, so that it is put in as large a box as possible to gain the advantage of having a large volume of liquid. The top and bottom slates measure seven inches by seven inches, and the distance between them is seven inches. The inner coil is wound on first, and the loop which constitutes the end of the winding is brought up to a suitable position for adjustment. The insulation of the heavy copper connectors is by means of ebonite.
§ 120. Platinum Iridium.
Platinum 90 per cent, iridium 10 per cent. This material was prepared in some quantity at the cost of the French Government, and distributed for test about 1886. Klemencic got some of it as representing Austria, and found it behaved very like the platinum silver alloy just discussed. The temperature coefficient is, however, higher than for platinum silver (0.00126 as against 0.00027). The mechanical properties of the alloy are, however, much better than those of the silver alloy; and in view of the experience with B. A. standards above quoted, it remains an open question whether, on the whole, it would not be the better material for standards, in spite of its higher price. Improvements in absolute measurements of resistance, however, may render primary standards superfluous.
§ 121. Manganin.
Discovered by Weston--at all events as to its application to resistance coils. A glance at the diagram will exhibit its unique properties, on account of which it has been adopted by the Physikalisch Technischen Reichsanstalt for resistance standards. The composition of the alloy is copper 84 per cent, manganese 12 per cent, nickel 4 per cent, and it is described as of a steel-gray colour. Unfortunately it is apt to oxidise in the air, or rather the manganese it contains does so, so that it wants a very perfect protection against the atmosphere.
Like German silver, manganin changes in resistance on winding, and coils made of it require to be artificially aged by heating to 150° for five hours before final adjustment. The annealing cannot be carried out in air, owing to the tendency to oxidation. The method adopted by St. Lindeck (at all events up to 1892) is to treat the coil with thick alcoholic shellac varnish till the insulation is thoroughly saturated, and then to bake the coil as described. The baking not only anneals the wire, but reduces the shellac to a hard and highly insulating mass.
Whether stresses of sufficient magnitude to produce serious mechanical effects can be set up by unequal expansion of wire and shellac during heating and cooling is not yet known, but so far as tested (and it must be presumed that the Reichsanstalt tests are thorough) no difficulty seems to have been met with. In course of time, however, probably the best shellac coating will crack, and then adieu to the permanency of the coil! This might, of course, be obviated by keeping the coil in kerosene, which has no action on shellac, but which decomposes somewhat itself.
The method of treatment above described suffices to render coils of manganin constant for at least a year (in 1892 the tests had only been made for this time) within a few thousands per cent. Manganin can be obtained in sheets, and from this material standards of 10-2, 10-3, and 10-4 ohms are made by soldering strips between stout copper bars, and these are adjusted by gradually increasing their resistance by boring small holes through them. The solder employed is said to be "silver."
Mr. Griffiths (Phil. Trans. vol. clxxxiv. [1893], A, p. 390) has had some experience with manganin carrying comparatively heavy currents, under which circumstances its resistance when immersed in water was found to rise in spite of the varnish which coated it. Other experiments in which the manganin wire was immersed in paraffin oil did not exhibit this effect, though stronger currents were passed.
On the whole, manganin appears to be the best material for coil boxes and "secondary" resistance standards. Whether it is fit to rank with the platinum alloys as regards permanency must be treated as an open question.
§ 122. Other Alloys.
The following tables, taken from the work of Feussner and St. Lindeck, Zeitschrift fuer Instrumenten Kunde, 1889, vol. ix. p. 233, together with the following notes, will suffice.
§ 123. Nickelin.
This is only German silver with a little less zinc, a little more nickel, and traces of cobalt and manganese. It behaves like German silver, but is an improvement on the latter in that all the faults of German silver appear upon a reduced scale in nickelin.
§ 124. Patent Nickel.
Practically a copper nickel alloy, used to some extent by Siemens and Halske. It stands pretty well in the same relation to nickelin as the latter does to German silver. After annealing as for manganin it can be made into serviceable standards which do not change more than a few thousandths per cent. I have not come across a statement of its thermo-voltage against copper.
§ 125. Constantin.
Another nickel copper alloy containing 50 per cent of each constituent. It appears to be a serviceable substance, having a temperature coefficient of 0.003 per cent per degree only, but an exceedingly high thermo-voltage, viz. 40 micro-volts per degree against copper.
1 2 3 4 5 6 7 8 German Nickelin made Rheo- Patent Nickel Manga- Nickel Silver by Obermaier tane nese Manga- Dia- Dia- Dia- Dia- Copper nese meter meter meter meter Copper 1.0mm 0.1mm 0.6mm 1.0mm
Copper 60.16 61.63 54.57 53.28 74.41 74.71 70 73
Zinc 25.37 19.67 20.44 16.89 0.23 0.52 ... ...
Tin ... ... ... ... trace ... ...
Nickel 14.03 18.46 24.48 25.31 25.10 24.14 ... 3
Iron 0.30 0.24 0.64 4.46 0.42 0.70 ... ...
Cobalt trace 0.19 ... ... trace trace ... ...
Mang- trace 0.18 0.27 0.37 0.13 0.17 30 24 anese.
99.86 100.37 100.40 100.31 100.24 100.24 ... ...
Specific resistance 30.0 33.2 44.8 52.5 34.2 32.8 100.6 47.7
Temperature coefficient 0.00036 0.00030 0.00033 0.00041 0.00019 0.00021 0.00004 0.00003
The specific resistance is in microhms, i.e. 10-6 ohms per cubic centimetre, and the temperature coefficient in degrees centigrade.
126. Nickel Manganese Copper.
I can find no other reference with regard to this alloy mentioned by Lindeck. Nicholls, however (Silliman's Journal [3], 39, 171, 1890), gives some particulars of alloys of copper and ferromanganese. The following table is taken from Wiedemann's Beiblatter (abstract of Nicholl's paper, 1890, p. 811). All these alloys appear to require annealing at a red heat before their resistances are anything like constant.
Let x be percentage of copper, then 100--x is percentage of "ferromanganese."
Values of x. 100 99.26 91 .88 86.98 80.4 70.65
Specific resistance with respect to copper (? pure) 1 1.19 11.28 20.4 27.5 45.1
Temperature coefficient per degree x 10^6(hard) 3202 2167 138 16 22 -24
Ditto (soft) ... ... 184 80 66 21
If nickel is added, alloys of much the same character are obtained, some with negative temperature coefficients--for instance, one containing 52.51 per cent copper, 31.27 per cent ferromanganese, and 16.22 nickel.
A detailed account of several alloys will be found in a paper by Griffiths (Phil. Trans. 1894, p. 390), but as the constants were determined to a higher order of accuracy than the composition of the material--or, at all events, to a higher degree of accuracy than that to which the materials can be reproduced--there is no advantage in quoting them here.