The Electric Bell
The electric bell consists of two solenoids wound in opposite directions on two soft iron cores. One of the wires around the soft iron cores is connected to a switch and the other is connected to a bracket which supports a spring mounted soft iron armature.
The armature carries a light spring, to which is connected a small metal disc acting as a contact. The latter presses against the end of a contact screw which is then connected to the battery.
When the switch is pressed current flows through the circuit and the iron cores become magnetised, inducing opposite polarity in the armature. The cores attract the armature and the contacts separate, breaking the circuit. The magnetism disappears and the armature returns to it's original position. The circuit is remade and the process repeats. Consequently, the armature vibrates and the hammer strikes the gong.