Electric Bell

Electric Bell 

a device consisting of an electromagnet, an armature with a hammer, and a cup (bell). When a button connected to the electric bell is pressed, thereby closing a DC supply circuit to the electromagnet, the armature is attracted and its hammer strikes the bell cup. Upon being attracted to the electromagnet the armature opens the contacts of a circuit breaker in the supply circuit and is returned to its position by the action of a spring. A basic drawback is the sparking at the contacts, which creates considerable radio interference. When supplied from a 127–220 V AC system, an electric bell is supplied through a transformer built into the bell’s housing, so that the voltage at the button does not exceed 12 V (to meet safety specifications). The AC electric bell, which operates on the principle of a polar relay, has no contacts and utilizes the change in the direction of the magnetic field to move the hammer. There are also resonant polar types, which are employed in selective signaling systems (for example, in telephone systems).