The electromagnet and the electric bell

The main parts of an electric bell are an electromagnet, an armature beside it, a hammer fixed to the armature, a gong cup and a contact screw. When voltage is applied to the input terminals, current flows from the positive pole through the contact screw, armature and electromagnet winding to the negative pole. Current in the winding makes the electromagnet pull the armature, and the hammer on it strikes the gong. As soon as the armature is pulled, contact with the screw breaks, the current stops, the electromagnet loses its magnetism and the spring-like armature returns to its start position. When contact is made again current flows again and the process repeats rapidly, so the hammer hits the gong again and again and it rings continuously. So the bell is a device that breaks and makes its own circuit. Adjusting the gap between the contact screw and armature tunes the sound. The school model works from a 4.5–6 V battery; touching the inside or open contacts of a 220 V house bell is dangerous, and only a qualified master repairs it. A spark may appear when contact breaks, so no flammable items are kept near the contacts.