The function of an electric bell relies on two things: the opening and closing of a circuit and an electromagnet. When power is running through the electric bell's circuit, an electromagnet draws a mounted metal clapper towards the bell, striking it. However, when the clapper is pulled by the electromagnet, it breaks the bell's circuit. No longer drawn in by the electromagnet, the clapper then falls back to its resting position. Back at the resting position, the clapper again completes the circuit, causing the current to pass back through the electromagnet, which once again draws the clapper toward the bell.
Building the Bell
Wrap the magnet wire around one of the nails and sand off any insulation from either end of the wire. This will be our electromagnet.
Slide the electromagnet through a hole in one upright of the wooden U.
Using a wood screw, mount the metal clapper into the bottom interior of the wooden U in such a way that the hammerhead can freely move back and forth. Then, connect one wire end from the electromagnet to the bottom of the clapper.
Slide the other nail through a hole drilled into the other upright of the wooden U and attach a standard electric wire to it.
Attach the standard wire from the nail to either the positive or negative end of the two AA batteries connected in series. Attach the free wire from the electromagnet to the other end of the batteries.
Mount the entire setup so that it lies flat on the wooden board and allows room for the bell to be mounted near the head of the clapper. Adjust both the nail and the electromagnet so that the clapper rapidly oscillates back and forth.
Mount the bell on the wooden board so that the clapper repeatedly strikes it when the circuit with the electromagnet is complete.