An electric bell depends for its action on the fact that a piece of iron wound with insulated wire becomes a magnet and will attract another piece of iron just so long as an electric current is allowed to travel through the wire.

The instant the current ceases, the magnetism also ceases, and the attracted piece of iron (termed the armature) is no longer held in contact.

The general construction of an electric bell is shown in Fig.1. MM are coils of insulated wire wound on soft iron cores. A is a soft iron armature mounted on a flat spring so that it is normally kept a slight distance away from the soft iron cores. S is a brass screw with a platinum tip touching a platinum disc on a spring attached to the armature.

When the push button P is pressed down, its two brass springs touch each other, the current from the battery cell B then flows through the wire W, through the push P, through the coils MM, along A to the platinum disc, out at S, which touches this disc, and back to the battery.

The instant this is done the current causes the iron cores to become magnets, they attract A, which then breaks contact at S. The spring mounting of A causes it to jump back to its first position, S then touches the platinum disc again, the current flows as before, and the armature is again attracted only to break contact with S and fly back.

This continual making and breaking of the circuit keeps up as long as the push is pressed, a ball mounted on A by means of a rod strikes against the gong G causing a continuous ringing of the bell. The wires leading between the bell, battery cell and push must all be insulated, that is, covered with cotton, rubber, etc., which prevents the leakage of current should two wires cross each other. Copper wire is mostly used for circuits indoors, the details of the kind and size of wire will be given later on.

The main parts of an electric bell circuit are then—the battery to supply the electric current; the circuit, or wires, to carry this current; a push, or circuit breaker, to control the current flow; and a bell to utilize the current.