The bell is realy easy in principle. You need a current to flow through a coil which is wound around a ferromagnetic core. When the switch finishes the circuit, the coil produces a magnetic field. The coil will change electric energy to magnetic and starts to pull the metal arm closer by a force. This force makes the arm, that is on a spring, move and hit the gong. When this happens, the circuit is broken, because the metal arm is used to complete the circuit. Since the metal arm is housed on a spring, it will return to its starting position, close the circuit again and the whole process will strart to happen all over again. This will go on until somebody stops holding the button down. When the curcuit is broken by the switch, this whole repeating proces will cease to happen. And the blissful silence shall take my house over once more.