CHAPTER III.

ON ELECTRIC BELLS AND OTHER SIGNALLING APPLIANCES.

[S] 41. An electric bell is an arrangement of a cylindrical soft iron core, or cores, surrounded by coils of insulated copper wire. On causing a current of electricity to flow round these coils, the iron becomes, _for the time being_, powerfully magnetic (see [S] 13). A piece of soft iron (known as the _armature_), supported by a spring, faces the magnet thus produced. This armature carries at its free extremity a rod with a bob, clapper or hammer, which strikes a bell, or gong, when the armature, under the influence of the pull of the magnet, is drawn towards it. In connection with the armature and clapper is a device whereby the flow of the current can be rapidly interrupted, so that on the cessation of the current the iron may lose its magnetism, and allow the spring to withdraw the clapper from against the bell. This device is known as the "contact breaker" and varies somewhat in design, according to whether the bell belongs to the _trembling_, the _single stroke_, or the _continuous ringing_ class.

[S] 42. In order that the electric bell-fitter may have an intelligent conception of his work, he should _make_ a small electric bell himself. By so doing, he will gain more practical knowledge of what are the requisites of a good bell, and where defects may be expected in any he may be called upon to purchase or examine, than he can obtain from pages of written description. For this reason I reproduce here (with some trifling additions and modifications) Mr. G. Edwinson's directions for making an electric bell:--[10]

_How to make a bell._--The old method of doing this was to take a piece of round iron, bend it into the form of a horse-shoe, anneal it (by leaving it for several hours in a bright fire, and allowing it to cool gradually as the fire goes out), wind on the wire, and fix it as a magnet on a stout board of beech or mahogany; a bell was then screwed to another part of the board, a piece of brass holding the hammer and spring being fastened to another part. Many bells made upon this plan are still offered for sale and exchange, but their performance is always liable to variation and obstruction, from the following causes:--To insure a steady, uniform vibratory stroke on the bell, its hammer must be nicely adjusted to move within a strictly defined and limited space; the least fractional departure from this adjustment results in an unsatisfactory performance of the hammer, and often a total failure of the magnet to move it. In bells constructed on the old plan, the wooden base is liable to expansion and contraction, varying with the change of weather and the humidity, temperature, etc., of the room in which the bells are placed. Thus a damp, foggy night may cause the wood to swell and place the hammer out of range of the bell, while a dry, hot day may alter the adjustment in the opposite direction. Such failures as these, from the above causes alone, have often brought electric bells into disrepute. Best made bells are, therefore, now made with metallic (practically inexpansible) bases, and it is this kind I recommend to my readers.

[Footnote 10: "Amateur Work."]

_The Base_, to which all the other parts are fastened, is made of 3/4 in. mahogany or teak, 6 in. by 4 in., shaped as shown at Fig. 17, with a smooth surface and French polished. To this is attached the metallic base-plate, which may be cut out of sheet-iron, or sheet-brass (this latter is better, as iron disturbs the action of the magnet somewhat), and shaped as shown in Fig. 18; or it may be made of cast-iron, or cast in brass; or a substitute for it may be made in wrought-iron, or brass, as shown in Fig. 19. I present these various forms to suit the varied handicrafts of my readers; for instance, a worker in sheet metal may find it more convenient to manufacture his bell out of the parts sketched in Figs. 17, 18, 20^A, 21, 23, 24^A, and 25; but, on the other hand, a smith or engineer might prefer the improved form shown at Fig. 31, and select the parts shown at Figs. 20^A, 22, 19, choosing either to forge the horse-shoe magnet, Fig. 20, or to turn up the two cores, as shown at Fig. 21 (A), to screw into the metal base, Fig. 21 B, or to be fastened by nuts, as shown at Fig. 19. The result will be the same in the end, if good workmanship is employed, and the proper care taken in fixing and adjusting the parts. A tin-plate worker may even cut his base-plate out of stout block tin, and get as good results as if the bell were made by an engineer. In some makes, the base-plate is cut or stamped out of thick sheet-iron, in the form shown by the dotted lines on Fig. 18, and when thus made, the part A is turned up at right angles to form a bracket for the magnet cores, the opposite projection is cut off, and a turned brass pillar is inserted at B to hold the contact screw, or contact breaker ([S] 41).

The _Magnet_ may be formed as shown at Fig. 20, or at Fig. 20^A. Its essential parts are: 1st. Two soft iron cores (in some forms a single core is now employed); 2nd. An iron base, or yoke, to hold the cores together; 3rd. Two bobbins wound with wire. The old form of magnet is shown at Fig. 20. In this form the cores and yoke are made out of one piece of metal. A length of round Swedish iron is bent round in the shape of a horseshoe; this is rendered thoroughly soft by annealing, as explained further on. It is absolutely essential that the iron be very soft and well annealed, otherwise the iron cores retain a considerable amount of magnetism when the current is not passing, which makes the bell sluggish in action, and necessitates a higher battery power to make it work (see [S] 14). Two bobbins of insulated wire are fitted on the cores, and the magnet is held in its place by a transverse strip of brass or iron secured by a wood screw passing between the two bobbins. The size of the iron, the wire, the bobbins, and the method of winding is the same as in the form next described, the only difference being that the length of the iron core, before bending to the horse-shoe form, must be such as to allow of the two straight portions of the legs to be 2 in. in length, and stand 1-3/8 apart when bent. We may now consider the construction of a magnet of the form shown at Fig. 20^A. To make the cores of such a magnet, to ring a 2-1/2 in. bell, get two 2 inch lengths of 5/16 in. best Swedish round iron, straighten them, smooth them in a lathe, and reduce 1/4 in. of one end of each to 4/16 of an in., leaving a sharp shoulder, as shown at Fig. 21 A. Next, get a 2-in. length of angle iron, drill in it two holes 1-3/8 apart, of the exact diameter of the turned ends of the cores, and rivet these securely in their places; this may be done by fastening the cores or legs in a vice whilst they are being rivetted. Two holes should be also bored in the other flange to receive the two screws, which are to hold the magnet to the base, as shown at Fig. 21 B. The magnet is now quite equal to the horse-shoe form, and must be made quite soft by annealing. This is done by heating it in a clear coal fire to a bright red heat, then burying it in hot ashes, and allowing it to cool gradually for a period of from 12 to 24 hours; or perhaps a better guide to the process will be to say, bury the iron in the hot ashes and leave it there until both it and they are quite cold. The iron must be brought to a bright cherry red heat before allowing it to cool, to soften it properly, and on no account must the cooling be hurried, or the metal will be _hard_. Iron is rendered hard by hammering, by being rapidly cooled, either in cold air or water, and hard iron retains magnetism for a longer time than soft iron. As we wish to have a magnet that will only act as such when a current of electricity is passing around it, and shall return to the state of a simple piece of unmagnetised iron when the current is broken, we take the precaution of having it of soft iron. Many bells have failed to act properly, because this precaution has been neglected, the "residual" (or remaining) magnetism holding down the armature after contact has been broken. When the magnet has been annealed, its legs should be polished with a piece of emery cloth, and the ends filed up level and smooth. If it is intended to fasten the cores into the base-plate, this also should be annealed, unless it be made of brass, in which case a thin strip of soft iron should connect the back ends of the two legs before they are attached to the brass base (an iron yoke is preferable, as it certainly is conducive to better effects to have a massive iron yoke, than to have a mere strip as the connecting piece). It will also be readily understood and conceded that the cores should be cut longer when they are to be fastened by nuts, to allow a sufficient length for screwing the ends to receive the nuts. The length and size of the legs given above are suitable for a 2-1/2 in. bell only; for larger bells the size increases 1/16 of an inch, and the length 1/4 of an inch, for every 1/2 in. increase in the diameter of the bell.

The _Bobbins_, on which the wire that serves to carry the magnetising current is to be wound, next demand our attention. They may be turned out of boxwood, ebony, or ebonite, or out of any hard wood strong enough and dense enough to allow of being turned down thin in the body, a very necessary requirement to bring the convolutions of wire as near the coil as possible without touching it. Some amateurs use the turned ends of cotton reels or spools, and glue them on to a tube of paper formed on the cores themselves. If this tube be afterwards well covered with melted paraffin wax, the plan answers admirably, but of course the bobbins become fixtures on the magnets. There are some persons who are clever enough to make firm bobbins out of brown paper (like rocket cases), with reel ends, that can be slipped off and on the magnet cores. To these I would say, "by all means at your command, do so if you can." The size of the bobbins for a 2-1/2 in. bell should be: length 1-3/4 in., diameter of heads 3/4 of an in., the length increasing 1/4 of an in. and the diameter 1/8 of an in. for every additional 1/2 in. in the diameter of the bell. The holes throughout the bobbins should be of a size to fit the iron cores exactly, and the cores should project 1/8 of an inch above the end of the bobbins when these are fitted on. The wire to be wound on the bobbins is sold by all dealers in electrical apparatus. It is copper wire, covered with cotton or with silk, to ensure insulation. Mention has already been made of what is meant by insulation at [S] 3, but, in order to refresh the reader's memory, Mr. G. Edwinson's words are quoted here. "To insulate, as understood by electricians, means to protect from leakage of the electric current, by interposing a bad conductor of electricity between two good conductors, thus insulating[11] or detaching them from electric contact."

[Footnote 11: _Insula_ in Latin means an island, hence an electrified body is said to be insulated when surrounded by non-conductors, as an island by the sea.]

The following list will enable my readers to see at a glance the value of the substances mentioned here as conductors or insulators, the best conductors being arranged from the top downwards, and the bad conductors or insulators opposed to them in similar order, viz., the worst conductors or best insulators being at the top:--

_Conductors._ _Insulators._ Silver. Paraffin Wax. Copper. Guttapercha. Iron. Indiarubber. Brass. Shellac. All Other Metals. Varnishes. Metallic Solutions. Sealing Wax. Metallic Salts. Silk and Cotton. Wet Stone. Dry Clothing. Wet Wood. Dry Wood. Oil, Dirt and Rust.

See also the more extended list given at [S] 5 for a more complete and exact classification.

It will be seen, on reference to the above, that copper is a good conductor, being excelled by silver alone in this respect; and that silk and cotton are bad conductors. When, therefore, a copper wire is bound round with silk or with cotton, even if two or more strands of such a covered wire be superimposed, since these are electrically separated by the non-conducting covering, no escape of electricity from one strand to the other can take place, and the strands are said to be insulated. If the copper wire had been coiled _naked_ round a bobbin, each convolution touching its neighbour, the current would not have circled round the whole length of the coils of wire, but would have leapt across from one coil to the other, and thus the desired effect would not have been obtained. A similar result, differing only in degree, would occur if a badly insulating wire were used, say one in which the covering had been worn in places, or had been badly wound, so as to expose patches of bare copper wire. If the insulation of a wire be suspected, it should be immersed in hot melted paraffin wax, and then hung up to drain and cool. The size of wire to be used on a 2-1/2 in. bell should be No. 24 B. W. G., the size falling two numbers for each 1/2 in. increase in the diameter of the bell. In these wires the higher the number, the finer the size, No. 6 being 1/5 and No. 40 being 1/200 of an inch in diameter. Silk-covered wire has an advantage over cotton-covered wire, inasmuch as the insulating material occupies less space, hence the convolutions of wire lie closer together. This is important, as the current has less effect on the iron if removed further from it, the decrease being as the _square_ of the distance that the current is removed from the wire. Magnets coiled with silk-covered wire admit also of better finish, but for most purposes cotton-covered wire will give satisfaction, especially if well paraffined. This wire must be wound on the bobbins, from end to end regularly, with the coils side by side, as a reel of cotton is wound. This may be done on a lathe, but a little practice will be necessary before the inexperienced hand can guide the wire in a regular manner. If, however, the spool of wire have a metal rod passed up its centre, and this be held in the hand at a distance of a foot or more from the bobbin on the lathe, the wire will almost guide itself on, providing the guiding hand be allowed to follow its course. With a little care, the wire for these little magnets may be wound entirely by hand. Before commencing to wind on the bobbins, just measure off 8 in. of the wire (not cutting it off) and coil this length around a pencil, to form a small coil or helix. The pencil may then be withdrawn from the helix thus formed, which serves to connect the wire with one of the points of contact. This free end is to be fastened outside the bobbin by a nick in the head; or the 1/8 in. length, before being formed into a helix, may be pushed through a small hole made on the head of the bobbin, so that 8 in. project _outside_ the bobbin, which projecting piece may be coiled into a helix as above described. The wire should now be wound exactly as a reel of cotton is wound, in close coils from end to end, and then back again, until three layers of wire have been laid on, so that the coiling finishes at the opposite end to that at which it began. To prevent this uncoiling, it should be fastened by tying down tightly with a turn or two of strong silk. The wire should now be cut from the hank, leaving about 2 in. of free wire projecting at the finishing end of each bobbin. In cases where many bobbins have to be wound, either for bells, for relays, or for indicator coils, a device similar to that illustrated at Fig. 21 A may be employed. This _electric bobbin winder_ consists in a table which can be stood on a lathe or near any other driving wheel. Two carriers, C C, somewhat similar to the back centre and poppet head of a lathe, hollow inside, and furnished with a spring and sliding piston spindle, stand one at each end of this table. The sliding spindle of the one carries at its extremity a pulley, A, by means of which motion can be transmitted from the band of the driving wheel. The sliding spindles, B B, are fitted with recesses and screws, H H H H, by means of which the temporary wooden cores, or the permanent iron cores, of the bobbins can be held while the bobbins are being wound. The bobbin is placed as shown at D; a flat piece of metal, E, hinged at G, presses against the bobbin, owing to the spring F. The centre figure shows details of the carrier, C, in section. At the bottom is shown the spool of wire on a standard L. The wire passes from this spot between the two indiarubber rollers, M M, on to the bobbin D.

When the bobbins have been wound, they may be slipped over the magnet cores. They should fit pretty tightly; if they do not, a roll of paper may be put round the magnet cores, to ensure their not slipping when the bell is at work. The helix ends of the bobbins should stand uppermost, as shown at Fig. 22 A. A short length of the lower free ends of wire (near the base or yoke) should now be bared of their covering, cleaned with emery paper, twisted together tightly, as shown at Fig. 22 B, soldered together, and any excess of wire cut off with a sharp pair of pliers. To prevent any chance electrical leakage between this bared portion of the wire and the iron, it should be carefully coated with a little melted guttapercha, or Prout's electric glue.

Of course, if the operator has any skill at winding, he may wind both bobbins with one continuous length of wire, thus avoiding joins, taking care that the direction of the winding in the finished coils be as shown at Fig. 22 B; that is to say, that the wire from the _under_ side of one bobbin, should pass _over_ to the next in the same way as the curls of the letter [rotated S].

The part that next claims our consideration is the _armature_, with its fittings. The armature is made out of 5/16 square bar iron, of the best quality, soft, and well annealed, and filed up smooth and true. The proportionate length is shown at Figs. 23 and 24; and the size of the iron for other bells is regulated in the same ratio as that of the cores. Two methods of making and attaching the springs and hammers are shown. Fig. 24 shows the section of an armature fitted with back spring and contact spring in one piece. This is cut out of hard sheet-brass, as wide as the armature, filed or hammered down to the desired degree of springiness, then filed up true on the edges. It may be attached to the iron of the armature, either by soldering, by rivetting, or by means of two small screws. Rivetting is, perhaps, the best mode, as it is not liable to shake loose by the vibration of the hammer. The spring at its shank end may be screwed or rivetted to the bracket. Mr. Edwinson considers this the better form of contact spring. The other form is made in two pieces, as shown at Fig. 23, where two strips of hard brass are cut off, of the width of the armature, and the edges filed. A slot is then cut in the back end of the armature to receive the two brass strips, and these are soldered into it. The top strip is then bent back over the armature to form the contact-spring, the other strip being soldered or rivetted to a small bracket of angle brass. In either case a short rod of stout hard brass wire is rivetted or screwed into the free end of the armature, and to the end of this rod is screwed or soldered the metal bead, or bob, which forms the hammer or "clapper" of the bell. The next portion to be made is the contact pillar, or bracket, with its screw, as shown at Fig. 25. This may either be a short stout pillar of 1/4 in. brass rod, about 1 in. high, tapped on one side to receive the screw, which should be fitted with a back nut; or it may, as shown in the figure, be made out of a stout piece of angle brass. The exact size and length of the screw is immaterial; it must, however, be long enough to reach (when put in its place behind the contact spring) the spring itself, and still have a few threads behind the back nut to spare. The screw should be nicely fitted to the pillar, and the lock nut should clench it well, as when once the adjustment of the parts is found which gives good ringing, it is advisable that no motion should take place, lest the perfection of ringing be interfered with. Some makers use a "set screw" at the side of the pillar wherewith to hold the contact screw; others split the pillar and "spring" it against the contact screw; but, all things considered, the back nut gives the greatest satisfaction. When the bell is in action, a tiny spark is produced at every make and break of contact between the contact spring and this screw. This spark soon corrodes the end of the screw and the back of the spring if brass alone is used, as this latter rusts under the influence of the spark. To prevent this, a piece of platinum must be soldered or rivetted to the spring, at the point where the screw touches, as shown at Fig. 26, and also at the extremity of the contact screw itself. It is better to rivet the platinum than to solder it, as the platinum is very apt to absorb the solder, in which case it rusts quickly, and the goodness of the contact is soon spoiled, when the bell ceases to ring. To rivet the platinum piece on to the spring, as shown at Fig. 26, it is only needful to procure a short length of No. 16 platinum wire, say 1/8 in., then, having drilled a corresponding hole at the desired spot in the contact spring, put the platinum wire half way through the hole, and give it one or two sharp blows on an anvil, with a smooth (pened) hammer.

This will at once rivet it in its place, and spread it sufficiently to make a good surface for contact. The screw must likewise be tipped with platinum, by having a small hole bored in the centre of its extremity, of the same diameter as the platinum wire, which must then be pushed in, and rivetted by hammering the end, and burring the sides of the screw. Whichever method be adopted, care must be taken that the platinum tip on the screw and the speck on the contact spring are adjusted so as to touch exactly in their centres. It will be hardly worth while for the amateur to cast or even turn up his own bells (which are generally of the class known as clock gongs), as these can now be procured so cheaply already nickelled (see Fig. 28). The bell must be adjusted on its pillar (see Fig. 29^A), which is itself screwed into a hole in the base-plate, where it is held by a nut. The adjustment of the bell is effected by placing it over the shoulder of the pillar, and then clenching it down by screwing over it one or other of the nuts shown at Fig. 29. The bell should clear the base, and should be at such a height as to be struck on its edge by the hammer or clapper attached to the armature, Figs. 23 and 24. We still need, to complete our bell, two binding screws, which may take either of the forms shown at Fig. 27; and an insulating washer, or collar, made of ebonite or boxwood, soaked in melted paraffin, to prevent the contact pillar (Fig. 25) making electrical contact with the metal base. The best shape to be given to these washers is shown at Fig. 30. They consist in two thin circlets of wood or ebonite, that will just not meet when dropped, one on the one side, and one on the other of the hole through which the shank of the contact pillar passes when set up on the base-plate. If a wooden base be used below the metal base-plate, then only one washer, or collar, need be used--that is, the one _above_--since the screw of the pillar will pass into the wood, and this is not a conductor. If the metal base alone be used, both washers must be employed, and a small nut (not so large as the washer) used to tighten up and hold the pillar firm and immovable in its place opposite the contact spring.

Having now all the parts at hand, we can proceed to fit them together, which is done as follows:--The bell pillar, with its bell attached, is fastened by its shank into the hole shown near B, Fig. 17, where it is screwed up tight by the square nut shown at Fig. 29 _c_. In the same manner, we must fasten the contact pillar, or bracket, shown at Fig. 24 A. Whichever form be used, we must take great care that it be insulated from metallic contact with the metal base-plate by washers, as shown at Fig. 30 (similar washers must be used for the two binding screws if the _whole_ base-plate be made in metal). This being done, the metal frame, Fig. 18, is put in position on the wooden base, as shown at Fig. 17, and screwed down thereto by the screws indicated at _s s s_. The magnet may then be screwed down to the metal frame as shown. The small bracket of angle brass marked B, in Figs. 23 and 24, is next screwed into its place; that is, in such a position that the armature stands squarely facing the poles of the electro-magnet, but not quite touching them (say 1/16 of an inch for a 2-1/2 in. bell). In setting up this and the contact pillar, the greatest care must be taken that the platinum tip of the contact screw, Fig. 25, should touch lightly the centre of the platinum speck at the back of the spring, Figs. 23 and 24, shown full size at Fig. 26.

The free ends of the helically coiled electro-magnet wires should now be inserted into short lengths of small indiarubber tubing (same as used for feeding bottles), the extremities being drawn through and 1 in. of the copper wire bared of its covering for the purpose of making good metallic contact with the connections. One of these ends is to be soldered, or otherwise metallically connected, to the angle brass carrying the armature, spring and clapper, the other being similarly connected with the left-hand binding-screw, shown at Fig. 17. Another short length of wire (also enclosed in rubber tubing) must be arranged to connect the contact screw pillar Fig. 17, with the right-hand binding-screw. When this has been done, we may proceed to test the working of the bell by connecting up the binding screws with the wires proceeding from a freshly-charged Leclanch['e] cell. If all have been properly done, and the connections duly made, the armature should begin to vibrate at once, causing the "bob," or hammer, to strike the bell rapidly; that is, provided the platinum tipped screw touches the platinum speck on the contact spring. Should this not be the case, the screw must be turned until the platinum tip touches the platinum speck. The armature will now begin to vibrate. It may be that the clapper runs too near the bell, so that it gives a harsh, thuddy buzz instead of a clear, ringing sound; or, possibly, the clapper is "set" too far from the bell to strike it. In either case a little bending of the brass wire carrying the clapper (either from or towards the bell, as the case may dictate) will remedy the defect. It is also possible that the armature itself may have been set too near, or too far from the electro-magnet. In the latter case, the clapper will not vibrate strongly enough, in the former the vibration will be too short, and the clapper may even stick to the poles of the electros, especially if these have not been carefully annealed. A little bending of the spring, to or from the magnets, will remedy these deficiencies, unless the distance be very much too great, in which case the bending of the spring would take the platinum tip out of the centre of the platinum speck.

[S] 43. Having thus constructed an efficient electric bell we may proceed to study its action and notice some of the defects to which it may be subject. In the first place, if we connect up the bell with the battery as shown in Fig. 17, viz., the left-hand binding-screw with the wire proceeding from the carbon of the Leclanch['e], and the right-hand screw with the wire from the zinc, then, if the platinum tipped screw touches the platinum speck, at the back of the contact spring, a current of electricity flows from the left-hand binding-screw all round the coils of the electro-magnets, passes along the contact spring and platinum speck, thence to the platinum tipped screw along the short length of wire to the right-hand binding-screw, whence it returns to the zinc element of the battery, thus completing the circuit. The current, in thus passing around the electro-magnet cores, converts them, _pro tem._, into a powerful magnet (see [S] 13); consequently, the armature, with its contact spring and hammer, is pulled towards the electro-magnets and at the same time gives a blow to the bell. Now, if instead of having the platinum speck attached to a flexible spring, it had been attached bodily to the rigid iron armature, directly the electro-magnets felt the influence of the current, the platinum speck would have also been pulled out of contact with the platinum screw, therefore the electro-magnet cores would have _immediately_ lost their magnetism (see [S] 13, last five lines). This would have been disadvantageous, for two reasons: 1st, because the _stroke_ of the hammer would have been very short, and consequently the ring of the bell very weak; and, 2nd, because, as even the softest iron requires some appreciable time for the electric current to flow round it to magnetise it to its full capacity, it would need a much greater battery power to produce a given stroke, if the contact were so very short. The use of an elastic contact spring is, therefore, just to lengthen the time of contact. But the electro-magnets, even when the flexible spring is used, do actually pull the platinum speck out of contact with the platinum screw. When this takes place, the circuit is broken, and no more current can flow round the electro-magnets, the spring reasserts its power, and the contact is again made between the contact screw and contact spring, to be again rapidly broken, each break and make contact being accompanied by a correspondingly rapid vibration of the armature, with its attendant clapper, which thus sets up that characteristic rapid ringing which has earned for these bells the name of trembling, chattering, or vibrating bells.

[S] 44. From a careful consideration of the last two sections it will be evident that the possible defects of electric bells may be classed under four heads: viz., 1st, Bad contacts; 2nd, Bad adjustment of the parts; 3rd, Defective insulation; 4th, Warpage or shrinkage of base. We will consider these in the above order. Firstly, then, as to bad contacts. Many operators are content with simply turning the terminal wires round the base of the binding-screws. Unless the binding-screws are firmly held down on to the wires by means of a back nut, a great loss is sure to occur at these points, as the wires may have been put on with sweaty hands, when a film of oxide soon forms, which greatly lowers the conductivity of the junction. Again, at the junction points of the wires with the contact angle brass and contact pillar, some workmen solder the junctions, using "killed spirits" as a flux. A soldered contact is certainly the best, electrically speaking, but "killed spirits," or chloride of zinc, should never be used as a flux in any apparatus or at any point that cannot be washed in abundance of water, as chloride of zinc is very _deliquescent_ (runs to water), rottens the wire, and spoils the insulation of the adjacent parts. If solder be used at any parts, let _resin_ be used as a flux. Even if any excess of resin remain on the work, it does no harm and does not destroy the insulation of any of the other portions. Another point where bad contact may arise is at the platinum contacts. Platinum is a metal which does not rust easily, even under the influence of the electric spark given at the point of contact. Therefore, it is preferred to every other metal (except, perhaps, iridium) for contact breakers. Platinum is an expensive metal, the retail price being about 30s. an ounce, and as it is nearly twice as heavy as lead (Lead 11. Platinum 21.5) very little goes to an ounce. For cheap bells, therefore, there is a great temptation to use some other white metal, such as silver, german silver, platinoid, etc.

The tip of the platinum screw may be tested for its being veritably platinum in the following mode: Touch the tip with the stopper of a bottle containing aquafortis, so as to leave a tiny drop on the extreme point of the suspected platinum. If it boils up green, or turns black, it is _not_ platinum; if it remains unaltered, it may be silver or platinum. After it has stood on the tip for a minute, draw it along a piece of white paper, so as to produce a streak of the acid. Expose the paper for a few minutes to sunlight. If the streak turns violet or pinky violet, the metal is _silver_; if the paper simply shows a slightly yellowish streak, the metal is platinum. The tip of the platinum screw must be carefully dried and cleaned after this trial before being replaced.

Secondly, as to bad adjustment. It is evident that the magnets and the armature must stand at a certain distance apart to give the best effects with a given battery power. The distance varies from 1/24 in. in the very smallest, to 1/8 in. in large bells. Sometimes (but only in very badly made instruments) the armature adheres to the poles of the electro-magnet. This is due to _residual_ _magnetism_ (see [S] 14), and points to hard or unannealed iron in the cores or armature. As a make-shift, this defect may be partially remedied by pasting a thin piece of paper over that surface of the armature which faces the poles of the electro-magnets. Another bad adjustment is when the platinum screw does not touch fairly on the centre of the platinum speck, but touches the spring or the solder. Rust is then sure to form, which destroys the goodness of the contact. To adjust the contact spring at the right distance from the platinum screw, hold the hammer against the bell or gong. The armature should now _just not touch_ the poles of the electro-magnet. Now screw up the platinum screw until it _clears_ the contact spring by about the thickness of a sheet of brown paper (say 1/50 of an inch). Let the hammer go, and notice whether the contact spring makes good contact with the platinum screw. This may be tried by the Leclanch['e] cell as well, so as to make sure of the character of the _ringing_. When this has been satisfactorily adjusted the back-nut or set screw may be tightened, to insure that the vibration of the hammer shall not alter the adjustment. It sometimes happens that the spring that bears the armature is itself either too strong (or set back too far) or too weak. In the former case, the electro-magnet cannot pull the armature with sufficient force to give a good blow; in the latter, the spring cannot return the armature, with its attendant contact spring, back to its place against the platinum screw. To ascertain which of these two defects obtains, it is only necessary, while the bell is in action, to press the spring lightly with a bit of wire, first _towards_ and then _away_ from the electro-magnets. If the ringing is improved in the first case, the spring is too strong; if improvement takes place in the latter case, the spring is too weak. The third source of inefficient action, defective insulation, is not likely to occur in a newly-made bell, except by gross carelessness. Still, it may be well to point out where electrical leakage is likely to occur, and how its presence may be ascertained, localized, and remedied. If the wire used to wind the electro-magnet be old, badly covered, or bared in several places in winding, it probably will allow the current to "short circuit," instead of traversing the whole length of the coils. If this be the case, the magnet will be very weak: the magnet of a 2-1/2-in. bell should be able to sustain easily a 1 lb. weight attached by a piece of string to a smooth piece of 1/2-in. square iron placed across its poles, when energized by a single pint Leclanch['e] cell. If it will not do this, the insulation may be suspected. If the wire has been wound on the bare cores (without bobbins), as is sometimes done, bared places in the wire may be touching the iron. This may be ascertained by connecting one pole of a bottle bichromate, or other powerful battery, with one of the wires of the electro-magnet coils, and drawing the other pole of the battery across the clean iron faces of the electro-magnet poles. If there is any leakage, sparks will appear on making and breaking contact. Nothing but unwinding and rewinding with a well covered wire can remedy these defects. The other points where the insulation may be defective are between the binding screws and the base, if this be all of metal; or between the contact spring block and the base, and the contact pillar. It is also probable (if the connecting wires have not been covered with indiarubber tubing, as recommended) that leakage may be taking place between these wires and some portion of the metal work of the base or frame. This must be carefully examined, and if any point of contact be observed, a little piece of Prout's elastic glue, previously heated, must be inserted at the suspected places. With regard to the binding screws, if they stand on the wooden base, their insulation (unless the base be very damp indeed) will be sufficiently good; but if the base is entirely metallic, then ebonite or boxwood washers must be used to insulate them from contact with the base-plate. With regard to the contact spring block and the platinum screw pillar, it is _permissible_ that one or the other should not be insulated from the base or frame; but one or the other _must_ be insulated by means of ebonite or other insulating washers. Personally, I prefer to insulate both; but in many really good bells only the platinum screw pillar is thus insulated. Any such leakage can be immediately detected by holding one pole of a powerful battery against the suspected binding-screw, or block, or pillar, and while in this position, drawing the other pole across some bare iron portion of the frame or metal base. Sparks will appear if there is any leakage.

The fourth defect--that is, warpage or shrinkage of the base--can only occur in badly-made bells, in which the entire base is of wood. A cursory examination will show whether the board is warped or swollen, or whether it has shrunk. Warping or swelling will throw the electro-magnet too far from the armature, or "set" the pillar out of place; shrinkage, on the contrary, will bring the parts too close together and jamb the magnets, the armature, and the contact pillar into an unworkable position.

[S] 45. Before quitting the subject of the defects of bells, it may not be out of place to mention that no bell that is set to do real work should be fitted up without a cover or case. The dust which is sure to accumulate, not to speak of damp and fumes, etc., will certainly militate against good contacts and good action if this important point be neglected. The cover or case generally takes the form of a shallow box, as shown at Fig. 32, and may be made from 1/4-in. teak, mahogany, or walnut, dovetailed together and well polished. It is fastened to the base in the same manner as the sides of a Dutch clock, by means of studs, hooks and eyes. At the bottom of the box is cut a slot, of sufficient width and length to admit the play of the hammer shank.

In the annexed table is given a general idea of the proportion which should be observed in the construction of bells of different sizes. It must be noted that if the bells are to be used at long distances from the battery, rather more of a finer gauge of wire must be employed to wind the magnets than that herein recommended, unless, indeed, _relays_ be used in conjunction with the bells.

[S] 46.--

TABLE

Showing proportions to be observed in the different parts of electric bells.

---------+---------+----------+--------+---------+---------- Diameter |Length of|Diameter |Length |Diameter | B. W. G. of | Magnet |of Magnet | of |of Bobbin| of Wire Bell. | Cores. | Cores. |Bobbin. | Head. |on Bobbin. ---------+---------+----------+--------+---------+---------- 2-1/2" | 2" | 5/16" | 1-3/4" | 3/4" | 24 3 | 2-1/4 | 3/8 | 2 | 7/8 | 24 3-1/2 | 2-1/2 | 7/16 | 2-1/4 | 1 | 22 4 | 2-3/4 | 1/2 | 2-1/2 | 1-1/8 | 22 4-1/2 | 3 | 9/16 | 2-3/4 | 1-1/4 | 20 5 | 3-1/4 | 5/8 | 3 | 1-3/8 | 18 5-1/2 | 3-1/2 | 11/16 | 3-1/4 | 1-1/2 | 16 6 | 3-3/4 | 3/4 | 3-1/2 | 1-5/8 | 16 6-1/2 | 4 | 13/16 | 3-3/4 | 1-3/4 | 16 7 | 4-1/4 | 7/8 | 4 | 1-7/8 | 16 7-1/2 | 4-1/2 | 15/16 | 4-1/4 | 2 | 14 8 | 4-3/4 | 1 | 4-1/2 | 2-1/8 | 14 8-1/2 | 5 | 1-1/16 | 4-3/4 | 2-1/4 | 14 9 | 5-1/4 | 1-1/8 | 5 | 2-3/8 | 14 9-1/2 | 5-1/2 | 1-3/16 | 5-1/4 | 2-1/2 | 14 10 | 5-3/4 | 1-1/4 | 5-1/2 | 2-5/8 | 14 10-1/2 | 6 | 1-5/16 | 5-3/4 | 2-3/4 | 12 11 | 6-1/4 | 1-3/8 | 6 | 2-7/8 | 12 11-1/2 | 6-1/2 | 1-7/16 | 6-1/4 | 3 | 10 12 | 6-3/4 | 1-1/2 | 6-1/2 | 3-1/8 | 10 ---------+---------+----------+--------+---------+----------

[S] 47. We can now glance at several modifications in the shape and mode of action of electric bells and their congeners. Taking Figs. 33 A and B as our typical forms of trembling bell, the first notable modification is one by means of which the bell is made to give a single stroke only, for each contact with the battery. This form, which is known as the "single stroke bell," lends itself to those cases in which it may be required to transmit preconcerted signals; as also where it is desired to place many bells in one circuit. Fig. 34 illustrates the construction of the single stroke bell. It differs from the trembling bell in the mode in which the electro-magnet is connected up to the binding screws. In the trembling bell, Fig. 33, the circuit is completed through the platinum screw pillar, to the binding screw marked Z, hence the circuit is rapidly made and broken as long as by any means contact is made with the battery, and the binding screws L and Z. But in the single stroke bell, Fig. 34, the wires from the electro-magnet are connected directly to the two binding screws L and Z, so that when contact is made with the battery, the armature is drawn to the poles of the electro-magnet, and kept there so long as the battery current passes. By this means, only one stroke or blow is given to the bell for each contact of the battery. Of course, directly the connection with the battery is broken, the spring which carries the armature and clapper flies back ready to be again attracted, should connection again be made with the battery. To regulate the distance of the armature from the poles of the electro-magnets, a set screw Q takes the place of the platinum screw in the ordinary form, while to prevent the hammer remaining in contact with the bell (which would produce a dull thud and stop the clear ring of the bell), a stop (_g_) is set near the end of the armature, or two studs are fixed on the tips of the poles of the electro-magnets. The mode of adjusting this kind of bell, so as to obtain the best effect, differs a little from that employed in the case of the trembling bell. The armature must be pressed towards the poles of the electro-magnets, until it rests against the stop or studs. A piece of wood or cork may be placed between the armature and the set screw Q, to retain the armature in this position, while the rod carrying the hammer or clapper is being bent (if required) until the hammer just clears the bell. If it touches the bell, a thud instead of a ring is the result; if it is set off too far, the ring will be too weak. The armature can now be released, by removing the wood or cork, and the set screw Q driven forwards or backwards until the best effect is produced when tested with the battery. The tension of the armature spring must be carefully looked to in these single stroke bells. If it is too strong, the blow will be weak; if too weak, the hammer trembles, so that a clear single stroke is not obtainable, as the spring _chatters_.

[S] 48. _The continuous ringing bell_ is the modification which next demands our attention. In this, the ringing action, when once started by the push,[12] or other contact maker, having been touched, continues either until the battery is exhausted, or until it is stopped by the person in charge. The great use of this arrangement is self-evident in cases of burglar alarms, watchman's alarms, etc., as the continuous ringing gives notice that the "call" has not received attention. The continuous ringing bell differs but little from the ordinary trembling bell. The chief difference lies in the addition of an automatic device whereby contact is kept up with the battery, even after the "push" contact has ceased. As it is desirable for the person in charge to be able to stop the ringing at will, without proceeding to the place where the "push" stands, so it is not usual to make the continuous ringing arrangement dependent on the "push," though, of course, this could be done, by causing it to engage in a catch, which would keep up the contact, when once made. Continuous ringing bells may be conveniently divided into two classes; viz., 1st, those in which a device is attached to the framework of the bell; which device, when once upset by the first stroke of the bell, places the bell in direct communication with the battery independent of the "push" or usual contact; and 2ndly, those in which a separate device is used, for the same purpose. This latter arrangement admits of the use of an ordinary trembling bell.

[Footnote 12: A "push," of which several forms will hereafter be described and figured, consists essentially in a spring carrying a stud, standing directly over, but not touching, another stud, fixed to a base. The lower stud is connected to one terminal of battery, the spring is connected to the bell. When the spring is pressed down, the two studs come into contact, the current flows, and the bell rings.]

Fig. 35 illustrates the action of bells of the first class. In the first place it will be noticed that there are three binding screws instead of two, as in the ordinary pattern, one marked C connected as usual with the carbon element of the battery; another marked L, which connects with line wire, and a third, Z, connected by means of a branch wire (shunt wire), proceeding from the zinc of the battery. It will be seen, that if the battery current is by means of the push caused to flow through the coils of the electro-magnets, the armature is attracted as usual by them, and in moving towards them, releases and lets fall the lever contact, which, resting on the contact screw, completes the circuit between Z and C, so that the bell is in direct communication with its battery, independently of the push. Hence the bell continues ringing, until the lever is replaced. This can be done, either by pulling a check string (like a bell-pull) attached to an eye in the lever, or by means of a press-button and counter-spring; as shown in Fig. 36, A and B.

In continuous ringing bells of the second class, a detent similar to that shown at Fig. 35 D is used, but this, instead of being actuated by the electro-magnet belonging to the bell itself, is controlled by a separate and entirely independent electro-magnet, which, as it may be wound with many coils of fine wire, and have a specially light spring for the armature, can be made very sensitive. This second electro-magnet, which serves only to make contact with a battery, is known as a _Relay_, and is extensively employed in many cases where it is desired to put one or more batteries into, or out of circuit, from a distance. The relay may be looked upon as an automatic hand, which can be made to repeat at a distant point contacts made or broken by hand at a nearer one. Fig. 37 shows this arrangement, attached to the same base board as the bell itself. On contact being made with the push, the current enters at C, circulates round the cores of the relay, thus converting it into a magnet. The armature _a_ is thereby pulled to the magnet, and in so doing releases the detent lever, which falls on the contact screw, thus at one and the same time breaking the circuit through the relay, and making the circuit through the bell magnets B B', back to the battery by Z. A second modification of this mode of causing an ordinary bell to ring continuously is shown at Fig. 38, the peculiar form of relay used therewith being illustrated at Fig. 39. Here, the relay is placed on a separate base board of its own, and could, if necessary, be thrown out of circuit altogether, by means of a _switch_,[13] so that the bell can be used as an ordinary bell or continuous action at will. It will be noticed that the relay has in this sketch only one core. But the delicacy of the action is not impaired thereby, as the armature, by means of the steel spring _s_, is made to form part and parcel of the magnet, so that it becomes magnetised as well as the core, and is attracted with more force than it would be, if it were magnetically insulated. The battery current enters by the wires C and W, passes round the coils of the electro-magnet, and returns by Z. In so doing it energises the electro-magnet E, which immediately attracts its armature A. The forward movement of the armature A, releases the pivoted arm L, to which is attached a platinum-tipped contact prong P. This, it will be noticed, is in metallic connection with the pillar P', and with the base, and, therefore, through the wire W, with the battery. When the arm L falls, the contact prong completes the circuit to the bell, through the insulated pillar X. The relay is thus thrown out of the circuit at the same time that the bell is thrown in. A device similar to those illustrated at Fig. 36 can be employed to reset the arm L.

[Footnote 13: Described at [S] 61.]

A rather more complicated arrangement for continuous bell ringing is shown at Fig. 40. It is known as Callow's, and is peculiarly adapted to ringing several bells from one attachment, etc. Owing to the relay in this form being wound with two sets of wires, it takes a little more battery power; but this disadvantage is compensated by its many good points. The following description, taken from F. C. Allsop's papers in the _English Mechanic_, will render the working of Callow's attachment perfectly clear. "When the button of the push P is pressed, the current in the main circuit flows from the positive pole C of the battery D through the relay coil _a_, and thence by the wire _d_ and push P, to the zinc of the battery. This attracts the armature A of the relay R, closing the local bell circuit, the current flowing from C of the battery to armature A of the relay R, through contact post _p_, terminal L of the bell, through bell to terminal Z, and thence by the wire _g_ to the zinc of the battery. Part of the current also flows along the wire from the bell terminal L through the relay coil _b_ and switch W, to terminal Z of the bell, thus keeping the armature of the relay down, after the main circuit (through the push) has been broken; the bell continuing to ring until the shunt circuit is broken by moving the arm of the switch W over to the opposite (or non-contact) side. The bell can also be stopped by short circuiting the relay, which can be effected by an ordinary push. It will be seen that more than one bell can be rung from the same attachment, and the bell can, by moving the arm of the switch W, be made continuous ringing or not, at will. If the arm of the switch is moved over to the opposite side to which it is shown in the figure, the shunt circuit of the bell through the relay is broken, and the bell will ring only so long as the button of the push is kept in. This continuous arrangement is very convenient for front doors, etc., where trouble is experienced in securing immediate attention to the summons. Instead of being taken to the switch, as in Fig. 40, the two wires are taken to a contact piece fixed on the side of the door frame, and so arranged that when the door is opened, it either short circuits or breaks the shunt circuit: thus when the push is pressed, the bell rings until the door is opened, the continual ringing of the bell insuring prompt attention."

Mr. H. Thorpe, of 59, Theobald's Road, London, has devised a very ingenious arrangement for the continuous ringing of one or more bells for a stated period of time. This is shown at Fig. 40 A. It is set in action by pulling the ring outside the bottom of the core. The bell or bells then start ringing, as contact is established and kept up. The novelty lies in the fact that the duration of the contact, and consequently of the ringing, can be accurately timed from 5 seconds to 30 seconds, by merely inserting a pin at different holes in the rod, as shown. After the bells have rung the required time the instrument automatically resets itself.

[S] 49. The modifications we are now about to consider, differ from the ordinary bell, either in the shape or material of the bell itself, the relative disposition of the parts, or some structural detail; but not upon the introduction of any new principle. The most striking is certainly the Jensen bell, which is shown in section at Fig. 41.

According to Mr. Jensen's system of electric bells, the bell may take any desired form, that of the ordinary church bell being preferred, and the electro-magnetic apparatus is placed entirely inside the bell itself. To attain this end the electro-magnetic apparatus must be compact in form. A single electro-magnet has pole pieces at each end opposite to which an armature is suspended from a pivot and balanced by the hammer of the bell. At the back of the armature there may be a make and break arrangement, whereby a continuous succession of strokes is effected, or this may be omitted, in which case a single stroke is given when the contact with the battery is made, or both may be effected by separate wires, make contact with one wire, and a single stroke is struck; make it with the other and the current passes through the make and break and a succession of strokes is heard. When the contact-breaker is used, it is so arranged that a slight rub is caused at every stroke, so keeping the contact clean. The flexible break, with the ingenious wiping contact, is a great improvement over the ordinary screw, which often becomes disarranged.

The form of the magnet is such that a considerable degree of magnetic force is caused by a comparatively small battery power. The electro-magnetic apparatus being within the bell the latter forms a very effective and handsome shield for the former. Not only can the bell shield the electro-magnet from wet but the whole of the conducting wires as well.

The bell may be screwed to a tube through which passes the conducting wire, which makes contact with an insulated metallic piece in the centre of the top of the bell. Both the wire and the contact piece are as completely shielded from the weather as if within the bell itself.

The great point of departure is the discarding of the unsightly magnet box, and the hemispherical bell (_see_ Fig. 32), and substituting a bell of the Church type (see Fig. 42), and placing inside it an electro-magnet specially arranged. The inventors use a single solenoidal magnet of a peculiar construction, by which the armature is attracted by both poles simultaneously. By this means less than half the usual quantity of wire is required, thus reducing the external resistance of the circuit one half. Moreover the armature, besides being magnetised by induction, as acted on in the ordinary method of making electric bells, is by Messrs. Jensen's plan directly polarised by being in actual magnetic contact by the connection of the gimbal (which is one piece with the armature) with the core iron of their magnet. It is thus induced to perform the largest amount of work with the smallest electro-motive force. Instead of the armature and clapper being in a straight line attached to a rigid spring, which necessitates a considerable attractive power to primarily give it momentum, in the Jensen Bell the armature and hammer are in the form of an inverted [U], and being perfectly balanced from the point of suspension, the lines of force from a comparatively small magnetic field suffice to set this improved form of armature into instant regular vibration. By using a flexible break and make arrangement instead of the usual armature spring and set screw (at best of most uncertain action), it is found that a much better result is attained, and by this device the armature can be set much nearer the poles of the magnet with sufficient traverse of the hammer. This is in strict accordance with the law of inverse squares, which holds that the force exerted between two magnetic poles is inversely proportionate to the square of the distance between them, or, in other words, that magnets increase proportionately in their power of attraction as they decrease in the square of the distance. It will now be seen why these bells require so little battery power to ring them: firstly, the armature and hammer are so perfectly balanced as to offer but little resistance; secondly, the external resistance to the current is reduced; and thirdly, the best possible use is made of the electro-magnetic force at disposal.

[S] 50. The next modification which demands attention is the so-called "Circular bell." This differs from the ordinary form only in having the action entirely covered by the dome. Except, perhaps, in point of appearance, this presents no advantages to that. The bells known as "Mining bells" resemble somewhat in outward appearance the circular bell; but in these mining bells the action is all enclosed in strong, square teak cases, to protect the movement, as far as possible, from the effects of the damp. All the parts are, for the same reason, made very large and strong; the armature is pivoted instead of being supported on a spring, the hammer shank being long, and furnished with a heavy bob. The domes or bells are from 6 inches to 12 inches in diameter, and are generally fitted with _single stroke_ movement, so as to enable them to be used for signalling. The hammer shank, with its bob, and the dome, which stands in the centre of the case, are the only parts left uncovered, as may be seen on reference to Figs. 43 A and B, where the exterior and interior of such a bell are shown.

[S] 51. In the "Electric Trumpet," introduced by Messrs. Binswanger, of the General Electric Company, we have a very novel and effective arrangement of the parts of an electric bell and telephone together. This instrument, along with its battery, line and push, is illustrated at Fig. 44, where A is a hollow brass cylinder, in which is placed an ordinary electro-magnet similar to Figs. 20 or 20 A. At the front end, near B, is affixed by its edges a thin disc of sheet iron, precisely as in the Bell telephone,[14] and over against it, at B, is an insulated contact screw, as in the ordinary trembling bell. On the disc of sheet iron, at the spot where the screw touches, is soldered a speck of platinum. The wires from the electro-magnet are connected, one to the upper binding screw, the other to the brass case of the instrument itself, which is in metallic communication with the sheet iron disc. The return wire from the contact screw is shown attached to the insulated piece, and is fastened to another binding screw (not visible) on the base board. When contact is made with the battery, through the press or push, the magnet becomes energised, and pulls the iron disc or diaphragm towards it, causing it to buckle inwards. In doing this, contact is broken with the screw B; consequently the diaphragm again straightens out, as the magnet no longer pulls it. Again contact is made; when of course the same round of performances is continuously repeated. As the plate or diaphragm vibrates many hundreds of times per second, it sets up a distinctly musical and loud sound wave, not unlike the note of a cornet-a-piston, or a loud harmonium reed. With a number of these "trumpets," each diaphragm being duly tuned to its proper pitch, it would be possible to construct a novel musical instrument, working solely by electricity. The "pushes" need only take the form of pianoforte keys to render the instrument within the grasp of any pianoforte or organ player.

[Footnote 14: See "Electrical Instrument Making for Amateurs." Whittaker & Co. Second edition.]

[S] 52. Sometimes the gong or "dome" of the ordinary bell is replaced by a coil spring, as in the American clocks; sometimes quaint forms are given to the parts covering the "movement," so as to imitate the head of an owl, etc. But bells with these changes in outward form will not present any difficulty, either in fixing or in management, to those who have mastered the structural and working details given in this chapter.