=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. [Illustration: FIG. 238.--Diagram showing the parts of an electric doorbell.]