Electric bell
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, but they are now being widely replaced with electronic sounders.
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[edit] Interrupter bells
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]
The tone of the sound generated depends on the shape and size of the bell. Where several bells are installed together, they may be given distinctive rings by using different bells, even though the strike mechanisms are identical.
Another type, the single-stroke bell, has no interrupting contacts. The hammer strikes the gong once each time the circuit is closed. These are used to signal brief notifications, such as a shop door opening for a customer, rather than continuous warnings.
[edit] Electric buzzers
An electric buzzer is a similar mechanism to an interrupter bell, but without the resonant bell. They are quieter than bells, but adequate for a warning tone over a small distance, such as across a desktop.
With the development of cheap electronics from the 1970s onwards, most buzzers have now been replaced by electronic 'sounders'. These replace the electromechanical striker of a bell with an electronic oscillator and a loudspeaker, often a very cheap piezo transducer.
[edit] Telephone bells
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Where a bell is powered by AC a different design, the polarised bell, may be used. These have an armature containing a permanent magnet, so that this is alternately attracted and repelled by each half-phase and different polarity of the supply. 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. No contact breaker is required, so the bells are reliable for long service. For this reason they were widely used for telephone bells.[2]
[edit] Fire Alarm Bells
Fire alarm bells are divided into two categories: vibrating, and single-stroke. On a vibrating bell, the bell will ring continuously until the power is cut off. When power is supplied to a single-stroke bell, the bell will ring once and then stop. It will not ring again until power is turned off and on again. These were frequently used with coded pull stations.
[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.[3] Bells for industrial purposes may operate on other, higher, AC or DC voltages to match plant voltages or available standby battery systems. [4]
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[edit] See also
[edit] References
- ^ a b Frederick Charles Allsop Practical electric bell fitting: a treatise on the fitting-up and maintenance of electric bells and all the necessary apparatus Author E. & F. N. Spon, 1890 pp. 30-32
- ^ Kennedy, Rankin. "Chapter IV: Telephones". The Book of Electrical Installations. Vol III (Unknown - the 'lamp' cover ed.). Caxton. pp. 126—127.
- ^ Terrel Croft, Wilford Summers (ed), American Electrician's Handbook Eleventh Edition, Mc Graw Hill, 1987 ISBN 0-07-013932-6, sections 9.451 through 9.462
- ^ http://www.federalsignal-indust.com/CutSheets/CS_A4_A6_A10_500_600.pdf retrieved 2011 April 29 Bell manufacturer cut sheet showing 24 V AC/DC, 120/240 V AC/DC bells
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