Electric Signals.
—Electrical signaling devices for household use, in the form of bells and buzzers, are made in a great variety of forms and sizes to suit every condition of requirement. The vibrating mechanism of the doorbell is used in all other household signals except that of the magneto telephone. It is an application of the electromagnet, in which the magnetism is applied to vibrate a tapper against the rim of a bell.
A bell system consists of the gong with its mechanism for vibrating the armature, an electric battery or A.C. transformer connected to the magnet coils to form an electric circuit and a push button which serves to close the circuit whenever the bell is to be sounded. The bell system is an open-circuit form of apparatus; that is, the circuit is not complete except during the time the bell is ringing. By pressing the push button the circuit is closed and the electric current from the battery flows through the magnet and causes the tapper to vibrate. When the push button is released the circuit is broken and the circuit stands open until the bell is to be again used. The parts of the bell mechanism are shown in Fig. 238 where with the battery, the push button and the connecting wires is shown a complete doorbell outfit. These parts may be placed in different parts of the building and connected by wires as shown in the Fig. 239. The bell is located at R, in the kitchen. The battery is placed in the closet at B, the connecting wires are indicated by the heavy lines; they are secured to any convenient part of the wall and extend into the basement and are fastened to the joists. The wires terminate in the push button P, where they pass through the frame of the front door. The wires are secured by staples to keep them in place. Each wire is fastened separately to avoid the danger of short-circuiting. If both wires are secured with a single staple there is a possibility of the insulation being cut and a short produced across the staple.
Fig. 238.—Diagram showing the parts of an electric doorbell.
The battery B, in Fig. 238, is a single dry cell but more commonly it is composed of two dry cells joined in series. It is connected, as shown in the figure, to the binding posts P1 and P2 of the vibrating mechanism, the push button PB serving to make contact when the circuit is to be closed. When the button is pressed the circuit is complete from the + pole of the battery cell through the binding post P1, across the contact F, through the spring A, through the magnet coils M, across the binding post P2 and push button to the-pole of the cell. The vibration of the tapper is caused by the magnetized cores of the coils M. When the electric current flows through the coils of wire, the iron cores become temporary magnets. This magnetism attracts the iron armature attached to the spring A, and it is suddenly pulled forward with energy sufficient to cause the tapper to strike the gong. As the armature moves forward, the spring contact at F is broken and the current stops flowing through the magnet coils. When the current ceases to flow in the magnet coils, the cores are demagnetized and the armature is drawn back by the spring A to the original position. As soon as the contact is restored at F a new impulse is received only to be broken as before. In this manner the bell continues ringing so long as the push button makes contact. The screw at F is adjusted to suit the contact with the spring attached to the armature. The motion of the armature may be regulated to a considerable degree by this adjustment. When properly set the screw is locked in place by a nut and should require no further attention.
Fig. 239.—Example of an electric doorbell installation.
Electric bells vary in price according to design and workmanship. A bell outfit may be purchased complete for $1 but it is advisable to install a bell of better construction, as few pieces of household mechanism repay their cost in service so often as a well-made bell. The bell should be rigid, well-constructed, and the contact piece F should be adjustable. This part F, being the most important of the moving parts of the bell, is shown separately in Fig. 240. Only the ends of the magnet coils with their cores are shown in the figure. The contact is made at A, by the pressure of the spring against the end of the adjustable screw D. When the screw is properly adjusted it is locked securely in place by the nut G. The screw D is held with a screw-driver and the nut G forced into position to prevent any movement. If the screw is moved, so that contact is lost at A, the bell will not ring. In the better class of bells the point of the screw and its contact at A are made of platinum to insure long life. With each movement of the armature a spark forms at the contact which wears away the point, so that to insure good service these points must be made of refractory material.
Fig. 240.—Diagram of the vibrating mechanism used in buzzers and doorbells.