See the animation above right. The bell or
gong (B), which is often in the shape of a cup or half-sphere, is struck by a spring loaded arm with a metal ball on the end called a
clapper (A), 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 an
electric current is passed 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.
[1]
Another type, the single-stroke bell, has no interrupting contacts. The hammer strikes the gong once each time the circuit is closed.
[edit] Power sources
Electric bells are typically designed to operate on low
voltages of from 10 to 24 V
AC or
DC. Before widespread distribution of electric power, bells were necessarily powered by batteries, either wet-cell or dry-cell type.
[1] Bells used in early
telephone systems derived current by a
magneto generator cranked by the subscriber. In residential applications, a small bell-ringing
transformer is usually used to power the doorbell circuit. So that bell circuits can be made with low-cost wiring methods, bell signal circuits are limited in voltage and power rating.
[2]
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