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An electric bell is a mechanical bell that functions by means of an electromagnet.
Principle
In DC electric bells, when power is applied, current flows through the coil. The coil becomes an electromagnet, attracting the metal strip. This moves the clanger to hit the bell, but also breaks the circuit. The coil is no longer a magnet, so the clanger moves back. The circuit is thus restored. The process repeats continuously until the power is removed.
AC electric bells do not have interrupting contacts and their coils are powered directly by the source. Their hammers vibrate at same frequency as the frequency of voltage they are powered by. Lack of contacts makes them more reliable than DC bells.
Some electric bells have two cups which generate different tones. When the hammer goes in one direction, it hits one cup, when it moves back, it hits another cup. The sound of such two-tone electric bells is more pleasant.
Applications
Two early applications of the electric bell were the telephone and doorbell. Early telephones used electric bells to indicate that there was an incoming call. Doorbells were used by visitors to indicate their presence at the external door of a dwelling or business. Though still in use, the electric bell mechanisms in both telephones and doorbells now compete with non-mechanical noisemaking technologies including electronic oscillators and digitally recorded sounds played back through a speaker.
A common style of doorbell uses an AC solenoid coil and a plunger. When the doorbell button is depressed, the plunger is drawn into the solenoid and strikes a gong; a shading coil on the solenoid prevents the plunger from vibrating at the same frequency as the power supply. When the button is released, a spring retracts the plunger which then strikes a second gong, giving a two-tone sound. A variant has a second solenoid which is wired to the back door and only strikes one gong, allowing front or rear door callers to be identified.
See also
