Chapter 5

For measurements of currents from 10 amperes upward, there is no need to employ a complete coil as the deflecting agent; one half-coil or one strip passing close under the needle gives sufficient deflecting force, and thus the construction of the instrument is rendered extremely simple. The current, after entering at one of the flat electrodes, splits in two parts, each part passing round the winding of an electro magnet of horseshoe form, the similar poles of both magnets pointing toward each other and toward the needle. After traversing the winding, the current unites again, and passes through a metal strip close under the needle, and finally out of the instrument by the other electrode, which lies close under that at which the current entered, but is insulated from it by a sheet of fiber. The metal strip is set at an angle, to balance or overbalance, as may be preferred, the magnetic influence of the exciting coils. The effect of this overbalancing is shown in Fig. 5, where the full curve represents the current as a function of the deflection--obtained by comparison with a standard instrument--and the dotted curve shows what that relation between deflection and current would be if the law of tangents held good for these instruments. It will be seen that, about the middle of the scale, the dotted line coincides nearly with the full line, while at the extreme end of the scale the dotted line is higher. From this follows, that if we compare our indicator from which this curve was taken with any form of tangent instrument showing an equal angle of deflection at the medium reading, it will be seen that the needle of our indicator will be deflected to a greater angle at high readings than that of the tangent galvanometer. Consequently, the divisions on the scale will be widest apart in our instruments, which greatly facilitates high readings.

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SECONDARY BATTERIES.

The Consolidated Electric Light Company has now completed the secondary battery which has for some time engaged the attention of its officers, and their regular manufacture and use for electric lighting stations have been fairly entered upon. Among other places to which the batteries have been sent and put into work is Colchester, where the company has for some time had an installation at work, chiefly employing incandescent lamps. The battery consists of lead electrodes, anode and cathode being of the same character. They are constructed of narrow ribbons of lead, each element being made from long lengths of the ribbon about or nearly 0.20 in. width, rolled together into a flat cake like rolls of narrow webbing, as illustrated by the annexed diagram, Fig. 1, the greater part of the ribbon being very thin and flat; but intermediate thicker ribbons are also employed, as in Fig. 2, this thicker ribbon being corrugated as shown, and affording passage room for the circulation of the electrolyte. From four to eight coils of the plain ribbons are between every pair of corrugated ribbons. They are wound up together tightly, and pressed into the nearly rectangular form shown. The bar for suspending the coil plates so made in the cells is soldered to the coil. The object of this construction is of course to obtain large lead surface, and of course a much larger surface is so obtained than could be practically obtained from plain lead plates in the same compass. A battery thus made may be seen at the offices of the company, 110 Cannon Street.

A very ingenious device for cutting the battery out of circuit when charged as much as is thought desirable is used by the company. In a cell is an element which has a determined lower capacity than those in the rest of the battery. Over this element is placed a gas-tight chamber in which is a diaphgram, this diaphragm being of very flexible material placed in the cover of the box of cells. When charging has proceeded as long as is desirable, or proceeds too fast, hydrogen is evolved, and this collecting in the chamber referred to acts upon the diaphragm, and by means of a rod connected thereto, switches the current, which is supplied to an electro-magnet and by which circuit is made through the medium of mercury contacts. The object, of this is to save the battery from destruction by over-charging or charging by too large a current.--_The Engineer_.

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ACETYLENE FROM IODOFORM.

P. CAZENEUVE publishes in the _Comptes Rendus_ a new method for the preparation of acetylene, which consists in mixing iodoform intimately with moist and finely divided silver. An abundant evolution of acetylene takes place without heating. The reaction is represented by the following formula: 2CHI_{3} + 6Ag = C_{2}H_{2} + 6 AgI. The decomposition of the iodoform is hastened if the silver is mixed with finely divided copper, such as can be obtained by precipitating it from its sulphate by means of zinc.

Cazeneuve also observed that most metals which have any affinity for iodine will decompose iodoform in the presence of water, forming acetylene and an iodide of the metal. By the use of zinc he obtained a liquid having a pleasant ethereal odor, and a gas mixture that contained besides acetylene an iodine compound which burned with a purple-edged, fawn-colored flame.

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WHEN DOES AN ELECTRICAL SHOCK BECOME FATAL?

In this age of electricity and electric wires carrying currents of various intensity, the question of danger arising from contact with them has caused considerable discussion. An examination into the facts as they exist may therefore enlighten some who are at present in the dark.

To begin with, we often hear the question asked--why is it that certain wires carrying very large currents give very little shock, whereas others, with very small currents, may prove fatal to those coming in contact with them? The answer to this is--that the shock a person experiences does not depend upon the current _flowing in the wires_, but upon the current _diverted from them_ and _flowing through the body_.

The muscular contraction due to a galvanic current, which was first observed in the frog, gives a good illustration of the fact that it requires only a very minute current to flow through the muscles in order to contract them. Thus the simple contact of pieces of zinc and copper with the nerves generated current sufficient to excite the muscles--a current which would require a delicate galvanometer for its detection. What is true of the muscles of the frog holds good also for the human muscles; they too are very susceptible to the passage of a current.

In order to determine the current which proves fatal we need only to apply the formula which expresses Ohm's law, viz., C=E/R, or the current (ampere) equals the electromotive force (volt) divided by the resistance (ohm).

According to the committee of Parliament investigation, the electromotive force dangerous to life is about 300 volts; this then is the quantity, E, in the formula. There remains now only to determine the resistance in ohms which the body offers to the passage of the current. In order to obtain this, a series of measurements under different conditions were made. On account of the nature of the experiment a high resistance Thomson reflecting galvanometer was used, with the following results.

When the hands had been wiped perfectly dry, the resistance of the body was about 30,000 ohms; with the hands perspiring ordinarily it fell to 10,000 ohms; whereas when they were dripping wet it was as low as 7,000 ohms. Our readers can judge this resistance best when we state that the Atlantic cable offers a resistance of 8,000 ohms.

Taking an ordinary condition of the body, with the hands perspiring as usual, we would have the resistance equal to 10,000 ohms. Applying the two known quantities in the formula, we get:

C = (300 / 10,000) - (1 / 33.333+)

This means, therefore, that when the electromotive force or potential is great enough to send a current of 1/33 ampere through the body, fatal results will ensue. This current is so minute that it would deposit only about 6 _grains_ of copper in _one hour_, a grain being 1/7,000 of a pound.

Let us now compare these figures with some actual cases, taking as an example a system of incandescent lighting. In these systems the difference of potential between any two points of the circuit outside of the lamps does not exceed 150 volts. Taking this figure, therefore, it will be seen that under no circumstances can the shock received from touching these wires become dangerous--not even by touching the terminals of the dynamo itself; because in neither case can a current be driven through the body, sufficient to cause an excessive contraction of the muscles.

In a system of arc lighting, however, we have to deal with entirely different conditions; for, while in the incandescent system the adding of a lamp, which diminishes the resistance, requires no increase of electromotive force, the contrary is the case in the arc light system. Here every additional lamp added to the circuit means an increase in resistance, and consequent increase in electromotive force or potential. Taking for example a well known system of arc lighting, we find that the lamps require individually an electromotive force of 40 volts with a current of 10 amperes. In other words, the difference in potential at the two terminals of every such lamp is 40 volts. Consequently, if the circuit were touched in two places, including between them only one lamp, no injurious effects would ensue. If we touch the circuit so as to include two lamps between us, the effect would be greater, since the potential between those two points is 2 x 40 volts. We might continue in this manner touching the circuit until we had included about 7 or 8 lamps, when the shock would become fatal, since the point would be reached at which the difference of potential is great enough to send a dangerous current through the body.

Up to this point we have assumed that, while touching two points in the wire, the rest of the circuit is perfectly insulated, so that no current can leak, in other words, that the circuit is nowhere "grounded." If this is not the case we may, under suitable conditions, receive a shock by touching only _one_ point of the wire. This becomes clear by considering the current to leak from another spot of different potential, to pass through the ground and into the body; thus, on touching the wire the body virtually makes a connection between the two points of the circuit. In clear dry weather such leaks are insignificant; but in damp and rainy weather, and with poor insulation, they may rise to such a point at which it would be dangerous to touch the circuit even with one hand, the leaks being sometimes so great as to cause the lamps to burn in a fitful, desultory manner, and to go out entirely.

There is still another factor which enters into the discussion of the danger of electric light wires. This must be looked for in the fact that the physiological effects are greatest at the moment of the opening or the closing of the circuit; or in a closed circuit they are the more marked when the flow of current stops and starts, or diminishes and increases. In dynamo electric machines the current is not absolutely continuous or uniform, since the coils on the armature being separated a distance cause a slight break or diminution of the current between each. This break is so short that it does not interfere with the practical work for lighting; in some constructions, nevertheless, the distances apart is so great that, while not interfering with light, its effects upon the muscles are greatly increased over those of other constructions which give a more uniform current.

All these statements might lead to the conclusion that arc light wires are dangerous under any circumstances; but this is not the case. The first and only requisite is, that they be perfectly insulated. When thus protected accidents from them are impossible, and all mishaps that have occurred through them can be traced directly to the lack of insulation. Nevertheless, we would warn our readers against experimenting upon arc wires by actual trial, because unforeseen conditions might lead to disagreeable results.

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ROBERT CAUER'S STATUE OF LORELEI.

The statue of Lorelei, the mythical siren of the Rhine, represented in the annexed cut, which is taken from the _Illustrirte Zeitung_, was modeled by Robert Cauer, of Kreuglach on the Rhine. He was born at Dresden in 1831, and is the son of the well-known sculptor Emil Cauer, and a brother of the sculptor Karl Cauer.

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REDUCING AND ENLARGING PLASTER CASTS.

Ordinary casts taken in plaster vary somewhat, owing to the shrinkage of the plaster; but it has hitherto not been possible to regulate this so as to produce any desired change and yet preserve the proportions. Hoeger, of Gmuend, has, however, recently devised an ingenious method for making copies in any material, either reduced or enlarged, without distortion.

The original is first surrounded with a case or frame of sheet metal or other suitable material, and a negative cast is taken with some elastic material, if there are undercuts; the inventor uses agar-agar. The usual negative or mould having been obtained as usual, he prepares a gelatine mass resembling the hektograph mass, by soaking the gelatine first, then melting it and adding enough of any inorganic powdered substance to give it some stability. This is poured into the mould, which is previously moistened with glycerine to prevent adhesion. When cold, the gelatine cast is taken from the mould, and is, of course, the same size as the original. If the copy is to be reduced, this gelatine cast is put in strong alcohol and left entirely covered with it. It then begins to shrink and contract with the greatest uniformity. When the desired reduction has taken place, the cast is removed from its bath. From this reduced copy a cast is taken as usual. As there is a limit to the shrinkage of the gelatine cast, when a considerable reduction is desired the operation is repeated by making a plaster mould from the reduced copy, and from this a second gelatine cast is taken and likewise immersed in alcohol and shrunk. It is claimed that even when repeated there is no sacrifice of the sharpness of the original.

When the copy is to be enlarged instead of reduced, the gelatine cast is put in a cold water bath, instead of alcohol. After it has swollen as much as it will, the plaster mould is made as before. For enlarging, the mould could also be made of some slightly soluble mass, and then by filling it with water the cavity would grow larger, but it would not give so sharp a copy.

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STRIPPING THE FILM FROM GELATINE NEGATIVES.

We have frequent inquiries as to the best means of removing a gelatino-bromide negative from its glass support so that it can be used either as a direct or reversed negative, and it does not appear to be very generally known that about two years ago Mr. Plener described a method which answers well under all circumstances, whether a substratum has been used or not.

If a negative is immersed in extremely dilute hydrofluoric acid contained in an ebonite dish, say half a teaspoonful to half a pint of water, the film very soon becomes loosened, and floats off the glass, this circumstance being due to the solvent action which the acid exercises upon the surface of the plate as soon as it has penetrated the film. If the floating film be now caught upon a plate which has been slightly waxed, and it is allowed to dry on this plate, it will become quite flat and free from wrinkles. To wax the plate, it should be held before the fire until it is moderately hot, after which it is rubbed over with a lump of wax, and the excess is polished off with a piece of flannel. When the film is dry, it will leave the waxed glass immediately, if one corner is lifted by means of a penknife. The film will become somewhat enlarged during the above-described operation; but, by taking suitable precautions, this enlargement may be avoided. It is also convenient to prepare the hydrofluoric acid extemporaneously by the action of sulphuric acid on fluoride of sodium; and, in many cases, it is advisable to thicken up the film by an additional layer of gelatine.

The following directions embody these points. The negative, which must be unvarnished, is leveled, and covered with a layer of warm gelatine solution (one in eight) about as thick as a sixpence. This done, and the gelatine set, the plate is immersed in alcohol for a few minutes in order to remove the greater part of the water from the gelatinous stratum. The next step is to allow the plate to remain for five or six minutes in a cold mixture of one part of sulphuric acid with twelve parts of water, and in the mean time two parts of sodium fluoride are dissolved in one hundred parts of water, an ebonite tray being used. A volume of the dilute sulphuric acid equal to about one-fourth of the fluoride solution is next added from the first dish, and the plate is then transferred to the second dish, when the film soon becomes liberated. When this is the case, it is placed once more in the dilute sulphuric acid. After a few seconds it is rinsed in water, and laid on a sheet of waxed glass, complete contact being established by means of a squeegee, and the edges are clamped down by means of strips of wood held in position by American clips or string. All excess of sulphuric acid may now be removed by soaking the plate in methylated alcohol, after which it is dried. It is as well to add a few drops of ammonia to the last quantity of alcohol used.

The plate bearing the film negative is now placed in a warm locality, under which circumstances a few hours will suffice for the complete drying of the pellicular negative, after which it may be detached with the greatest ease by lifting the edges with the point of a penknife.--_Photo. News_.

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NEW ANALOGY BETWEEN SOLIDS, LIQUIDS, AND GASES,

By W. SPRING.

The author asks in the first place, What is the cause of the different specific gravities of one and the same metal according as it has been cast, rolled, drawn into wire, or hammered? Does the difference observed prove a real condensation of the matter under the action of pressure, or is it merely due to the expulsion by pressure of gases which have been occluded when the ingot was cast? According to well-known researches, metals such as platinum, gold, silver, and copper, which have been proved to occlude gases on fusion, and to let them escape, _incompletely_, on solidification, are precisely those which are most increased in their specific gravity by pressure. The author has submitted to pressures of about 20,000 atmospheres metals which possess this property, either not at all, or to a very trifling extent, and he finds that though a first pressure produces a slight permanent increase of density, its repetition makes little difference. Their density is found to have reached a maximum. Hence the density of solids, like that of liquids, is only really modified by temperature. Pressure effects no permanent condensation of solid bodies, except they are capable of assuming an allotropic condition of greater density. The author's former researches tend to show that solid matter, in suitable conditions of temperature, takes the state corresponding to the volume which it is compelled to occupy. Hence there is an analogy between the allotropic states of certain solids and the different states of aggregation of matter. Possibly the different forms of matter may be due to a single cause--polymerization. The limit of elasticity of a solid body is the critical moment when the matter begins to flow under the action of the pressure to which it is submitted, just as, e.g., ice at or below 0° may be liquefied by strong pressure. A brittle body is simply one which does not possess the property of flowing under the action of pressure.

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HYDROGEN AMALGAM.

Hydrogen, although a gas, is recognized by chemists as a metal, and when combined with any solid metal--as in the case known to electricians as the polarization of a negative element,--the compound may correctly be termed an alloy; while any compound of hydrogen with the fluid metal mercury may with equal correctness be termed an amalgam of hydrogen, or "hydrogen amalgam." The efforts of many chemists and mining engineers have for many years been devoted to a search for some effective and economical means for preventing the "sickening" of mercury and its consequent "flouring" and loss. Some sixteen or more years ago, Professor Crookes, F.R.S., discovered and, after a series of experiments, patented the use of an amalgam of the metal sodium for this purpose. He made the amalgam in a concentrated form, and it was added in various proportions to the mercury used for gold amalgamation. Water becoming present, it will readily be understood that the sodium, in being converted into the hydrate (KHO) of that metal, caused a rapid evolution of hydrogen. The hydrogen thus evolved was the excess over a certain proportion which enters into combination with the mercury. While the mercury retained the charge of hydrogen, the "quickness" of the fluid metal was preserved; but upon the loss of the hydrogen the "quickness" ceased, and the mercury was acted upon by the injurious components contained in the ore.

Since the introduction of the sodium amalgam, many attempts have been made, more especially in America, to overcome the tendency of mercury to "sicken" and lose its "quickness." The greater number of these efforts have been made by the use of electricity as the active agent in attaining this end; but such efforts have been generally of a crude and unscientific character. Latterly Mr. Barker, of the Electro-amalgamator Company, Limited, has introduced a system--already detailed in these pages--by which the mercury is "quickened." In his method the running water passing over the tables, or other apparatus of a similar character, is used as the electrolyte. In this arrangement, the mercury being the cathode, plates or wires of copper constituting anodes are brought into contact with the water passing over the mercury in each "riffle." Both the cathode and the anodes are, of course, maintained in contact with the poles of a suitable source of electrical supply. The current then passes from the copper anode through the running water to the mercury cathode, and so on to the negative pole of the electro-motor. As a consequence of this arrangement, hydrogen is evolved from the water, and has the effect of reducing any oxide or other detrimental compound of the metal; in other words, it "quickens" and prevents "sickening" of the fluid metal, and consequent "flouring" and loss. While the hydrogen is evolved at the cathode, oxygen enters into combination with the copper constituting the anodes. This to some extent impairs the conductivity of the circuit.

The latest process, however, is that of Mr. Bernard C. Molloy, M.P., which we have already characterized as highly scientific and effective, the production of a suitable amalgam being obtained under the most economical and simple conditions. This process has the advantage of producing not only a hydrogen amalgam, but also at will an amalgam of hydrogen combined with any metal electro-positive to this latter. Thus hydrogen potassium or hydrogen sodium can be obtained, as will be seen by the following description.