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Installing Electric Bells

bell, circuit, spring, armature, battery and cores

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INSTALLING ELECTRIC BELLS The modern building is not considered com plete unless it is equipped throughout with electric bells to be used for signaling from one position to another. Electric bell wiring, on account of being operated at a low voltage from batteries—which reduces the likelihood of fires caused by short circuits and bad contacts—is not governed by the Inspection Departments of cities, nor by the Fire Underwriters. This fact oftentimes results in installations being poorly made in order to save in first cost, though this practice, as a rule, is more expensive in the end, on account of poor service and cost of re pairs. Hence considerable care should be exer cised in the selection of materials and the method of installing wires and apparatus.

The various parts composing the bell circuit, no matter what the exact application may be, are: the bell, the push-button or device for closing or opening the circuit; the battery, which supplies electrical energy; and the con ductor or wire, which connects the different parts and forms the circuit.

The operation of the electric bell depends upon the following principle. If a current of electricity is passed through a coil of wire sur rounding an iron core, the core becomes magnetized and will attract other magnetic sub stances. Fig. 20 is a diagrammatic representa tion of the ordinary vibrating bell or buzzer. The circuit through the bell is as follows: from binding post Bl, through windings of coils M, and M2, to spring Sl, through armature A, to spring s, through contact-point C, to adjusting screw S to other binding post B.

Installing Electric Bells

When battery current is sent through coils M, and by closing the circuit of which they form a part, their cores are magnetized, and they attract the iron armature A, which carries the spring s. The armature, which is itself supported by the spring will then move over toward the iron cores until the contact at C is broken by the spring s leaving the point of the adjusting screw S. As soon as the contact is broken, current ceases to flow through the cir cuit, and the cores in the coils and are demagnetized, whereupon the armature, carried by the spring will fly back again and re establish the circuit at C, again remagnetizing the cores of and M2. The process is then

repeated, again and again. As a result, the armature is kept flying back and forth, being vibrated as long as there is battery connected to and B and the circuit is closed outside of the bell through the battery.

The armature A carries a tapper which strikes against the gong G. The contact-point C should be formed of platinum, as there is a small arc formed when circuit is broken, which eats away other metals very rapidly. All parts of the bell should be well insulated; and the bell should be mounted on a metal back instead of on wood, as the latter is likely to warp and throw the bell out of adjustment. The mechan ism of the bell should also be provided with a covering, as this prevents trouble due to dust collecting at the point of contact C.

Bells and buzzers operating on this principle are manufactured in a number of different forms, their difference being more mechanical than electrical.

The device employed in closing the circuit is what is termed a push-button. This is so con structed that the contacts in the button are held normally open by means of a spring S (see Fig. 21). W is a circular piece of fiber which supports S and S', the two terminals of the cir cuit, and which is held in place by means of metal ring R, fastened to the wall or other surface by screws. C is a metal cap that is screwed onto the ring R and holds the push P in place. This is a very good form of push button; but they are manufactured in various other forms and with different finishes.

The battery used in bell work should be of the open-circuit type, the circuit being normally open. Two cells are usually sufficient to operate the ordinary bell or buzzer. Good dry cells are very satisfactory for this class of work, on account of their cleanliness, as they cannot spill.

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