Electric bell
Encyclopedia
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 telephone
s, fire and burglar alarm
s, as school bells, doorbell
s, and alarms in industrial plants, but they are now being widely replaced with electronic sounders.
(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.
Another type, the single-stroke bell, has no interrupting contacts. The hammer strikes the gong once each time the circuit is closed.
s 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. 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.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 Bells for industrial purposes may operate on other, higher, AC or DC voltages to match plant voltages or available standby battery systems.
Bell (instrument)
A bell is a simple sound-making device. The bell is a percussion instrument and an idiophone. Its form is usually a hollow, cup-shaped object, which resonates upon being struck...
that functions by means of an electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
. When an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
is applied, it produces a repetitive buzzing or clanging sound. Electric bells have been widely used at railroad crossings
Level crossing
A level crossing occurs where a railway line is intersected by a road or path onone level, without recourse to a bridge or tunnel. It is a type of at-grade intersection. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion...
, in telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
s, fire and burglar alarm
Burglar alarm
Burglar , alarms are systems designed to detect unauthorized entry into a building or area. They consist of an array of sensors, a control panel and alerting system, and interconnections...
s, as school bells, doorbell
Doorbell
A doorbell is a signaling device typically placed near a door. Most doorbells emit a ringing sound to alert the occupant of the building to a visitor's presence, when the visitor presses a button. Many modern doorbells are electric — they are actuated by an electric switch...
s, and alarms in industrial plants, but they are now being widely replaced with electronic sounders.
Interrupter bells
See the animation above right. The bell or gongGong
A gong is an East and South East Asian musical percussion instrument that takes the form of a flat metal disc which is hit with a mallet....
(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
Clapper
A clapper may refer to one of the following:*Part of a bell*Clapper bridge*A sound-activated gadget called The Clapper*A character from the video game Donkey Kong Country 2: Diddy's Kong Quest*Clapboard used in film production...
(A), actuated by an electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
(E). In its rest position the clapper is held away from the bell a short distance by its springy arm. When an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
is passed through the winding of the electromagnet it creates a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
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.
Another type, the single-stroke bell, has no interrupting contacts. The hammer strikes the gong once each time the circuit is closed.
Telephone bells
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.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.Power sources
Electric bells are typically designed to operate on low voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s of from 10 to 24 V AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
or DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
. Before widespread distribution of electric power, bells were necessarily powered by batteries, either wet-cell or dry-cell type. Bells used in early telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
systems derived current by a magneto
Magneto
A magneto is a type of electrical generator.Magneto may also refer to:* Magneto , permanent magnetic alternating current rotary generator* ignition magneto, magnetos on internal combustion engines...
generator cranked by the subscriber. In residential applications, a small bell-ringing transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
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.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 Bells for industrial purposes may operate on other, higher, AC or DC voltages to match plant voltages or available standby battery systems.
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