How are electromagnets used in electric bells?
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An electric bell is a mechanical bell that functions by means of an electromagnet. When an electric current is applied, it produces a repetitive buzzing or clanging sound. Electric bells have been widely used at railroad crossings, in telephones, fire and burglar alarms, as school bells, doorbells, and alarms in industrial plants, since the late 1800s, but they are now being widely replaced with electronic sounders. An electric bell consists of one or more electromagnets, made of a coil of insulated wire around an iron core, which attract a springy iron armature with a clapper. When an electric current flows through the coils, the electromagnet creates a magnetic field which pulls the armature towards it, causing the clapper to strike the bell.
The interrupter bell evolved from various oscillating electromechanical mechanisms which were devised following the invention of the electromagnet by William Sturgeon in 1823.[5] One of the first was the oscillating electric wire invented by James Marsh in 1824.[6][5] This consisted of a wire pendulum dipping into a mercury trough, suspended between the poles of an electromagnet. When current was passed through the wire, the force of the magnet made the wire swing sideways, out of the mercury, which broke the current to the magnet, so the wire fell back. The modern electric bell mechanism had its origin in vibrating "contact breaker" or interrupter mechanisms devised to break the primary current in induction coils.[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. Other types were invented around that time by Siemens and Halske and by Lippens.[5] The polarized (permanent magnet) bell used in telephones, which appeared about 1860,[6] had its beginning in the polarized relay and telegraph developed by Werner Siemens around 1850.