The most widely used form is the interrupter bell, which is a mechanical bell that produces a continuous sound when current is applied. See animation, above. The bell or gong "(B)", which is often in the shape of a cup or half-sphere, is struck by a spring-loaded arm "(A)" with a metal ball on the end called a clapper, actuated by an electromagnet "(E)". In its rest position the clapper is held away from the bell a short distance by its springy arm. When the switch "(K)" is closed, an electric current passes from the battery "(U)" through the winding of the electromagnet. It creates a magnetic field that attracts the iron arm of the clapper, pulling it over to give the bell a tap. This opens a pair of electrical contacts "(T)" attached to the clapper arm, interrupting the current to the electromagnet. The magnetic field of the electromagnet collapses, and the clapper springs away from the bell. This closes the contacts again, allowing the current to flow to the electromagnet again, so the magnet pulls the clapper over to strike the bell again. This cycle repeats rapidly, many times per second, resulting in a continuous ringing.