Electric Bells and All About Them Part 10

If a _combined_ generator and bell be fitted at each end of a line, it becomes possible to communicate both ways; one terminal of each instrument must be connected to the line, and the other terminal on each to earth. A combined generator and bell is shown at Fig. 94. These instruments are always ready for use, require no battery or press-b.u.t.tons. The generator, Fig. 92, will ring seven bells simultaneously, if required, so powerful is the current set up; and by using a switch any number of bells, placed in different positions, can be rung, by carrying a separate wire from the switch to the bell.

[Ill.u.s.tration: Fig. 95.]

-- 75. Our work would not be complete unless we pointed out the means necessary to detect faults in our work. In order to localise faults, two things are requisite: first, a means of knowing whether the battery itself is working properly, that is to say, giving the due _amount_ of current of the right _pressure_, or E.M.F.; secondly, a means of detecting whether there is leakage, or loss of current, or break of circuit in our lines. Fortunately, the means of ascertaining these data can be all combined in one instrument, known as a linesman's galvanometer or detector, of which we give an ill.u.s.tration at Fig. 95.

It will be remembered (-- 10) that if a current be pa.s.sed over or under a poised magnetic needle, parallel to it, the needle is immediately deflected out of the parallel line, and swings round to the right or left of the current, according to the _direction_ of the current; likewise that the needle is deflected farther from the original position as the current becomes stronger. The deflections, however, are not proportionate to the strength of the current, being fairly so up to about 25 to 30 degrees of arc out of the original position, but being very much less than proportionate to the current strength as the needle gets farther from the line of current; so that a current of infinite strength would be required to send the needle up to 90. On this principle the detector is constructed. It consists of a lozenge-shaped magnetic needle, suspended vertically on a light spindle, carrying at one end a pointer, which indicates on a card, or metal dial, the deflection of the needle. Behind the dial is arranged a flat upright coil of wire (or two coils in many cases) parallel to the needle, along which the current to be tested can be sent. The needle lies between the front and back of the flat coil. The whole is enclosed in a neat wooden box, with glazed front to show the dial, and binding screws to connect up to the enclosed coil or coils. If the coil surrounding the needle be of a few turns of coa.r.s.e wire, since it opposes little resistance to the pa.s.sage of the current, it will serve to detect the presence of large _quant.i.ties_ of electricity (many amperes) at a low pressure; this is called a _quant.i.ty_ coil. If, on the other hand, the coil be one of fine wire, in many convolutions, as it requires more _pressure_, or E.M.F., or "intensity" to force the current through the fine high-resistance wire, the instrument becomes one fitted to measure the voltage or _pressure_ of the current, and the coil is known as the "intensity." If both coils are inserted in the case, so that either can be used at will, the instrument is capable of measuring either the quant.i.ty of electricity pa.s.sing, or the pressure at which it is sent, and is then known as a quant.i.ty and intensity detector. No two galvanometers give exactly the same deflection for the same amount of current, or the same pressure; the fitter will therefore do well to run out a little table (which he will soon learn by heart) of the deflection _his_ instrument gives with 1, 2, 3, 4, 5 and 6 Leclanche's _coupled in parallel_, when connected with the quant.i.ty coil. He will find the smaller sizes give less current than the larger ones. In testing the deflections given by the intensity coil, he must remember to couple his cells _in series_, as he will get no increase in _tension_ or _pressure_ by coupling up in parallel. In either case the cells should be new, and freshly set up, say, within 24 hours. As some of my readers may like to try their skill at constructing such a detector, I transcribe the directions given in "Amateur work" by Mr. Edwinson:--

-- 76. "Such an instrument, suitable for detecting the currents in an electric bell circuit, may be made up at the cost of a few shillings for material, and by the exercise of a little constructive ability. We shall need, first of all, a magnetised needle; this can be made out of a piece of watch spring. Procure a piece of watch spring two inches long, soften it by heating it to redness, and allowing it to cool gradually in a bed of hot ashes; then file it up to the form of a long lozenge, drill a small hole in the centre to receive the spindle or pivot, see that the needle is quite straight, then harden it by heating it again to a bright red and plunging it at once into cold water. It now has to be magnetised. To do this, rub it on a permanent horse-shoe, or other magnet, until it will attract an ordinary sewing needle strongly, or wrap it up in several turns of insulated line wire, and send many jerky charges of electricity from a strong battery through the wire. When it has been well magnetised, mount it on a spindle of fine hard wire, and secure it by a drop of solder. We will next turn our attention to the case, bobbin, or chamber in which the needle has to work. This may be made out of cardboard entirely, or the end pieces may be made of ivory or ebonite, or it may be made out of thin sheet bra.s.s; for our purpose we will choose cardboard. Procure a piece of stout cardboard 4-3/4 inches long by 2 inches wide, double it to the form of a Tndstickor match-box, and pierce it in exactly opposite sides, and in the centre of those sides with holes for the needle spindle. Now cut another piece of stout, stiff cardboard 2-3/4 inches long by 3/4 inch wide, and cut a slit with a sharp knife to exactly fit the ends of the case or body already prepared. The spindle holes must now be bushed with short lengths of hard bra.s.s or gla.s.s bugles, or tubing, made to allow the spindle free movement, and these secured in position by a little melted sh.e.l.lac, sealing-wax, or glue. The needle must now be placed in the case, the long end of the spindle first, then the short end in its bearing; then, whilst the case with the needle enclosed is held between the finger and thumb of the left hand, we secure the joint with a little glue or with melted sealing-wax. The end-pieces are now to be put on, glued, or sealed in position, and set aside to get firm, whilst we turn our attention to other parts. The case, 5 inches by 4 inches by 2 inches in depth, may be improvised out of an old cigar-box, but is best made of thin mahogany or teak, nicely polished on the outside, and fitted with a cover sliding in a groove, or hinged to form the back of the instrument.

The binding screws should be of the pattern known as the telegraph pattern, fitted with nuts, shown at Fig. 27. A small bra.s.s handle to be fitted to the top of the instrument, will also be handy. A circular piece of smooth cardboard 3-1/4 inches in diameter, with a graduated arc, marked as shown in Fig. 95, will serve the purpose of a dial, and a piece of thin bra.s.s, bent to the form of [box open down], will be required as a needle guard. The face of the dial may be a circular piece of gla.s.s, held in a bra.s.s ogee, or a hole the size of the dial may be cut in a piece of thin wood; this, glazed on the inside with a square of gla.s.s, may be made to form the front of the instrument over the dial. An indicating needle will also be required for an outside needle; this is usually made of watch spring, and nicely blued; but it may be made of bra.s.s or any other metal, one made of aluminium being probably the best on account of its lightness. It must be pierced with a hole exactly in the centre, so as to balance it as the beam of scales should be balanced, and should one end be heavier than the other it must be filed until they are equal.

We will now turn our attention to the coil.

Procure sixpennyworth of No. 36 silk-covered copper wire and wind three layers of it very evenly on the coil case or bobbin, being careful in pa.s.sing the needle spindle not to pinch it or throw it out of truth.

When this has been wound on, it will be found that one end of the wire points to the left and the other end to the right. These are destined to be connected to the under side of the binding screws shown on the top of Fig. 95. We therefore secure them to their respective sides with a touch of sealing wax, and leave enough wire free at the ends to reach the binding screws--say, about 6 inches. It is handy to have an additional coil for testing strong currents, and as this may be combined in one instrument at a trifle additional cost, we will get some line wire (No.

22) and wind six or eight turns of it around the coil outside the other wire; one end of this wire will be attached to an additional binding screw placed between the others, and the other end to left binding screw shown. The coil thus prepared may now be mounted in position. Pierce the board dial and the wood at its back with a hole large enough for the needle spindle to pa.s.s through from the back to the centre of the dial.

See that the thick end of the inside needle hangs downwards, then place the coil in the position it is intended to occupy, and note how far the needle spindle protrudes on the face of the dial. If this is too long, nip off the end and file it up taper and smooth until it will work freely in a hole in the needle guard, with all parts in their proper places. This being satisfactory, secure the coil in its place by sealing wax, or, better still, by two thin straps of bra.s.s, held by screws at each end, placed across the coil. Now clean the free ends of the coil wires, insert them under the nuts of the binding screws, fix the indicating needle on the end of the spindle outside, and see that it hangs in a vertical position with the inside needle when the instrument is standing on a level surface. Secure it in this position, screw on the needle guard, fasten on the gla.s.s face, and the instrument will be complete.

-- 77. Provided thus with an efficient detector, the fitter may proceed to test his work. In cases of _new installations_, take the wire off the carbon binding screw of the battery and attach it to one screw of the galvanometer (on the intensity coil side), next attach a piece of wire from the other binding screw of the galvanometer (the central one) so as to place the galvanometer in circuit. _There should be no movement of the needle_, and in proportion to the deflection of the needle, so will the loss or waste be. If loss is going on, every means must be used to remedy it. It is of the utmost importance to the effective working of the battery and bells that not the _slightest leakage_ or _local action_ should be allowed to remain. However slight such loss may be, it will eventually ruin the battery. Let damp places be sought out, and the wires removed from near them. Bad or injured coverings must also be looked for, such as may have been caused by roughly drawing the wires across angular walls, treading on them, or driving staples too tightly over them. Two or more staples may be touching, or two or more wires carelessly allowed to lie under one staple. The wire may have been bared in some places in pa.s.sing over the sharp edges of the zinc tube. The backs of the pushes should be examined to see if too much wire has been bared, and is touching another wire at the back of the push-case itself.

Or the same thing may be taking place at the junction with the relays or at the indicator cases. Should the defect not be at any of these places, the indicator should next be examined, and wire by wire detached (not cut) until the particular wire in which the loss is going on has been found. This wire should then be traced until the defect has been discovered. In testing underground wires for a loss or break, it will be necessary first to uncouple the _distant_ end, then to disconnect the other end from the instruments, and attach the wire going underground to the screw of the galvanometer. A piece of wire must then be taken from the other screw of the detector to the carbon end of the battery, and a second wire from the zinc end of the battery to the earth plate or other connection. Proceeding to that part of the wire where the injury is suspected, the wire is taken up, and a temporary earth connection having been made (water main, gas pipe, etc.), and by means of a sharp knife connected with this latter, the covering of the suspected wire penetrated through to the wire, so as to make a good connection between this suspected wire and the temporary earth plates. If, when this is done, the needle is deflected fully, the injury is farther away from the testing end, and other trials must be made farther on, until the spot is discovered. Wherever the covering of the wire has been pierced for testing, it must be carefully recovered, finished off with Prout's elastic glue, or gutta-percha, and made quite sound. The connections with the earth plates very frequently give trouble, the wires corrode or become detached from the iron pipes etc., and then the circuit is broken.

-- 78. When the fitter is called to localise defects which may have occurred in an installation which has been put up some time, before proceeding to work let him ask questions as to what kind of defect there is, and when and where it evinces itself. If all the bells have broken down, and will not ring, either the battery or the main go and return wires are at fault. Let him proceed to the battery, examine the binding screws and connected wires for corrosion. If they are all right, let the batteries themselves be tested to see if they are giving the right amount of current. This should be done with the quant.i.ty coil of the detector. Should the battery be faulty, it will be well to renew the zincs and recharge the battery, if the porous cell be still in good condition; if not, new cells should be subst.i.tuted for the old ones.

Should the battery be all right, and still none of the bells ring, a break or bad contact, or short circuit in the main wires near the battery may be the cause of the mischief. If some bell rings continuously, there must be a short circuit in the push or pushes somewhere; the upper spring of one of the pushes may have got bent, or have otherwise caught in the lower spring. _Pulls_ are very subject to this defect. By violent manipulations on the part of mischievous butcher or baker boys, the return spring may be broken, or so far weakened as not to return the pull into the "off" position. If, the batteries being in good order, any bell rings feebly, there is either leakage along its line, or else bad contact in the push or in the connections of the wires to and from the push. There should be platinum contacts at the ends of the push springs; if there are not, the springs may have worked dirty at the points of contact, hence the poor current and poor ringing. It is seldom that the bells themselves, unless, indeed, of the lowest quality, give any serious trouble. Still the set screw may have shaken loose (which must then be adjusted and tightened up), or the platinum speck has got solder on its face and therefore got oxidised. This may be sc.r.a.ped carefully with a penknife until bright. Or, purposely or inadvertently, no platinum is on the speck at all, only the solder. A piece of platinum foil should be soldered on the spot, if this is so. Or again (and this only in very bad bells), the electro-magnets being of hard iron, may have retained a certain amount of _permanent magnetism_, and pull the armature into permanent contact with itself. This can be remedied by sticking a thin piece of paper (stamp paper will do) over the poles of the magnet, between them and the armature. In no case should the fitter _cut_ or _draw up_ out of tubes, etc., any wire or wires, without having first ascertained that the fault is in that wire; for, however carefully joints are made, it is rare that the jointed places are so thoroughly insulated as they were before the cutting and subsequent joining were undertaken. To avoid as much as possible cutting uselessly, let every binding screw be examined and tightened up, and every length of wire, which it is possible to get at, be tested for continuity before any "slashing" at the wires, or furious onslaughts on the indicator be consummated.

In conclusion, I beg to record my thanks for the very generous a.s.sistance which I have received in the compilation of the foregoing pages from the electrical firms of Messrs. Blakey Emmot, Binsw.a.n.ger, Gent, Judson, Jensen, and Thorpe.

ADDENDUM.

THE Ga.s.sNER BATTERY.

Since the compilation of the foregoing pages, a _dry battery_, known by the above name, has found great favour with electric-bell fitters. Its peculiarity consists in the zinc element forming the outside cell. In this is placed the carbon, which is separated from the zinc by a thick paste or jelly made of gypsum and oxide of zinc. The cell can be placed in any position, works as well on its side as upright, is not subject to creeping, has an E.M.F. of about 15 volt, with an internal resistance of only 025 ohm in the round form, and 06 in the flat form. The Ga.s.sner dry battery polarizes much less quickly than the ordinary Leclanche. The only defects at present noticeable, are the flimsy connections, and the fact that the outer cases being _metal_ must be carefully guarded from touching one another. This can be effected by enclosing in a part.i.tioned _wooden box_.