Electric bell

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).

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.

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.

Where several bells are installed together, they may be given distinctive rings by using different size or shapes of gong, even though the strike mechanisms are identical.

Typical uses of buzzers and beepers include alarm devices, timers and confirmation of user input such as a mouse click or keystroke.

In practice, the armature is arranged symmetrically with two poles of opposite polarity facing each end of the coil, so that each may be attracted in turn.

[1] In some countries, notably the UK, the clapper struck two different sized bells in turn giving a very distinctive ring.

[clarification needed] Electric bells are typically designed to operate on low voltages of from 5 to 24 V AC or DC.

[4] The interrupter bell evolved from various oscillating electromechanical mechanisms which were devised following the invention of the electromagnet by William Sturgeon in 1823.

[5] Vibrating "hammer" interrupters were invented by Johann Philipp Wagner (1839) and Christian Ernst Neeff (1847), and was developed into a buzzer by Froment (1847).

[5][6] John Mirand around 1850 added a clapper and gong to make the standard electric bell[5][6] for use as a telegraph sounder.