In some households electric bells and their circuits are regarded as troublesome fitments. But the blame more often than not rests Avith the householder: and, when all is said, the bell operated by a pull and a complicated system of wires and cranks is liable to cause more vexation. Witness the two facts that in old houses where the mechanical system still prevails there are sure to be a number of bells or pulls out of action altogether; and that few, if any, mechanical bells are now fitted in new houses.
In order to be in a position to search for faults or carry out even the simplest installation one should have a general idea of how an electric bell acts, and how it is connected with push and battery.
The bell itself is mounted on an iron base, secured to a board. At the top are two terminals (T and T in the diagram). One of these is connected with the battery, the other with the push; and battery and push are also connected together.
A wire runs from the terminal T to an insulated pillar, r, in which is an adjustable screw. The tip of this screw makes contact normally with a stud riveted into a flat spring, s, which is attached at its other end to a part, B, of the iron base. The Spring also carries a block of iron, A. known as the armature, one face of which is close to the poles of a horseshoe electro-magnet, jr. From one end of the armature projects the hammer, n, for striking the bell.
When the button of the push is pressed in, two metallic parts, one connected with T, and the other with the battery, are brought together. Current then flows through the circuit, which includes p, the contact screw, s, and the windings of M. The magnet then becomes active, drawing A towards it, and causing II to strike the gong. The movement of A, however, separates s from the tip of the screw, and the circuit is broken, causing the magnet to become dead and A to move away under the influonco of the spring.
As soon as s makes contact with the pillar screw again the circuit is restored, and the armature flies forward again, causing a second blow on the bell; and this cycle is repeated many times a second, as long as the bell push is closed.
Faults in the Circuit
It will be evident fr. In the bell assembly itself there are four more connecting points, T, B, P, T. Looseness or corrosion at any connecting point, or the breakdown of the conducting wire between any two points, must obviously put the boil out of action.
Another place at which trouble may arise is the contact between the adjustable screw and s. If the screw be not screwed in far enough, s may not make proper contact with it when the armature falis back. On the other hand, if it is in too far, the spring may not clear it when pulled forward by the magnet, and the armature be held fast by the latter.
If trouble is suspected here, loosen the set-screw in p and advance or retract the sciew, while a second person keeps a push pressed in, until satisfactory results are obtained. If the bell has been in use a long time, it is advisable to brighten up the button on s and the tip of the screw with a few strokes of a jewellers file.
In the large majority of cases – probably 95 per cent, at least – bell trouble is due to the battery having become exhausted, or the connections with its terminals corroded. The latter defect is obvious to the eye, and can be corrected by scraping the contact parts bright and smearing them with vaseline.
A voltmeter will quickly show the con-dition of each cell. If one be not available, each cell in turn should be connected with the bell terminals. If it rings the bell feebly, or not at all, it mti6t be recharged or scrapped, according to the type.
If a bell operated through several circuits refuses to ring when one particular push is pressed, but responds strongly to the other pushes, it may be taken for granted that the circuit of which that push forms jiart has a fault in it – perhaps a broken wire, or a loose joint, or a dirty connection. The only course then is to examine the wires of the circuit carefully if they are accessible.
A search may reveal a joint which has been made by merely twisting the wires together, allowing looseness or corrosion to develop. Every twisted joint found should, after the wires have been well cleaned, be soldered, to ensure metallic contact, a non-corrosive flux being used.
Batteries for Electric Bells
The Leclanche cell, in either its wet or dry form, is used universally in ordinary household olcetric-bell installations. A wet cell consists of (a) a glass jar or con-tamer, in which are placed (b) a porous pot, into which are sealed a projecting carbon slab, capped with a cast-lead top and a screw terminal, and a mixture of carbon grains and manganese peroxide; (c) a thick zinc rod with a looped wire projecting from the top; and (d) a solution of sal ammoniac, two ounces to the pint.
The first three form the unit as it is brought from the electrician or ironmonger. To make it active, sal ammoniac is dissolved in hot water, in the proportions given, and when cool poured into the oell until it reaches the black band on the porous pot. After the cell has been standing a day, the level will have fallen considerably owing to absorption by the contents of the porous pot, and more solution must be added to restore it.
When the cell is fully active, it will give a pressure of I-J- volts, but – and this is an important point to note – the volume of current will decrease if the level of the solution falls through evaporation. It is therefore important to examine wet cells occasionally and to make good any loss with fresh solution.
The zinc rod will gradually be eaten away and will need replacing at long intervals. Otherwise the cell will do go d service if occasional cleaned out and provided with fresh solution, though after very long use the porous pot will have to be replaced.
The other type, the dry, is a modified form of the Lcclanche, sold ready for immediate use. It is not actually dry, as it contains liquid chemicals absorbed by solid material. But it exudes no moisture, being sealed up, and can be laid on its side or carried about without fear of spillage. Moreover, it is free from one defect of the wet type, that of corrosion at the terminals; and in many cases is preferable to the wet cell, though over a period of years its use may prove rather more expensive.
It should perhaps be noted that the wire projecting from a point in the top of a dry cell is the counterpart of that attached to the zinc rod of a wet cell; and tlxat a dry cell cannot be recharged but must be thrown away when exhausted.
The cells used, whether wet or dry, to make up a battery should in all cases be of the same size. The larger the cells are individually, the more work are they able to do before becoming exhausted.
Where circuits are long, several cells may hp. The battery then gives a voltage equal to that of a single cell multiplied by the number of cells, and has a correspondingly greater capacity for overcoming resistance.
In most cases two or three cells in scries suffice for the installation in a dwelling-house. Where there is only a single short circuit, a single cell only will be needed. Should this circuit be much used, the effect of a large cell is obtained by connecting small cells in parallel , all zincs and all carbons being grouped to two terminals.
Where cells are in series, the failure of any one reduces the pressure greatly, as of which are applied to the two brass springs of tbe push.
The longest circuit – or at least that with highest resistance – will give the lowest reading, and should be noted for testing condition of batteries. It may be added that the battery should be able to ring the bell strongly when this circuit is in use.
Installing Bell Circuits
Twin wire, that is, two wires separately insulated, and enclosed in a single wrapping, is in 4 – – -£ – 4 O the cell contributes no energy, while offering high resistance. It must therefore be either replaced or at least removed.
An advantage of the parallel and series-parallel arrangements is that the current is not blocked to the same extent by a faulty cell, since there is more than one path for it through the battery.
When a battery is running down, the longest circuit connected with it will probably become dead first, as having the highest resistance. Which circuit this is in a house can be found by inserting a pad between hammer and gong of the bell, unscrewing the cap of each push in turn, and testing with a voltmeter, the terminals most cases more conveniently and neatly run than two single wires. Whichever is selected, it should harmonise in colour as closely as possible with the surface to which it will be attached.
Before purchasing wire, run a string over the course selected for the circuit, following it as exactly as possible. This will give you the length of twin wire needed, or half the length of single wire. Allow a yard or so for connections and waste.
For fixing the wires, use electricians copper staples with padded tops where they will not be unsightly. If ordinary iron wire staples are employed, take care not to drive them in hard, or they may cut the insulation or break the wire.
Run wires in angles and other incon-spicuous places, such as grooves on kirtings or mouldings. Where there is a cellar, it may be convenient to lead the wires down into it and along the ceiling. Whatever be the course selected, the wire should not be exposed to damage and should be easily accessible for examination.
Suppose that an electric bell is required for the front door. A suitable place for the push will be on what is the right-hand door-post to a person outside the house. The bell will probably be heard best from all parts of the house if placed in the entrance hall. A bell and battery mounted on a single bracket can be purchased, and would simplify fixing.
A hole ½ inch in diameter is bored through the door-post at the point where the centre of the push will be. The twin wire is pushed through, and the wrapping removed for an inch or so, to allow half an inch of each of the two conductors to be freed of insulation. The ends are brought through the back of the push, bent into eyes and secured under the screws provided on the spring pieces. It may here be stated that a connection loop should be so bent and applied that tightening up the holding screw or nut tends to coil it closer and not open it.
When the wires have been connected, the push is screwed on to the post. On the farther side of the post form a loop of an inch or two of wire, to allow the push to be drawn forward, clear of the post, if need be, and anchor the wire beyond the loop with a staple.. The wiring is then continued till the bell is reached, when it is cut off at a point which will allow one of the conductors to reach a battery terminal, and the other a terminal on the bell. A piece of single wire is used to connect the remaining bell terminal with the other terminal of the battery.
If the battery is located some distance away from the bell, for convenience or other reason, the twin will be led from the push to the bell, one conductor cut, and the two cut ends be connected to the bell terminals. The twin is then carried on to the battery.
In cases where there are several circuits to a bell, twin wire may run from every push to a board on which there are two rows A and B, of terminals, all those of a row being metallically connected. The ends of each twin are clamped to an A and a B terminal; and the rows are connected with one terminal of the battery and the bell respectively. A wire joining the other terminals of the battery and bell completes the wiring.
The arrangement is practically tlie same where an indicator of the mechanical replacement pattern is used. The di, .gram supplied with the indicator will, however, show the necessary details.