Repeating coil

In telecommunication

Telecommunication

Telecommunication is transmission over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, drums, semaphore, flags or heliograph. In modern times, telecommunication typically involves the use of electronic devices such as the telephone,...

s, a repeating coil is a voice-frequency 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...

 characterized by a closed magnetic core

Magnetic core

A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical and electromechanical devices such as electromagnets, transformers, electric motors, and inductors. It is usually made of ferromagnetic metal such as iron, or...

, a pair of identical balanced

Balanced line

In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...

 primary (line

Line (electrical engineering)

In electrical engineering, a line is, more generally, any circuit of an electrical system. This electric circuit loop , consists of electrical elements connected directly by conductor terminals to other devices in series.- Overview :Consumer electronic devices concerned with audio often have a...

) windings, a pair of identical but not necessarily balanced secondary (drop) windings, and low transmission loss

Attenuation

In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, and X-rays are attenuated by lead....

 at voice frequencies

Voice frequency

A voice frequency or voice band is one of the frequencies, within part of the audio range, that is used for the transmission of speech.In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz...

. It permits transfer

Transfer

Transfer may refer to:*Call transfer*Decal, a sticker*Electron transfer*Heat transfer*Jacoby transfer, a bidding device in contract bridge*Knowledge transfer...

 of voice currents from one winding to another by magnetic induction

Electromagnetic induction

Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field....

, matches line and drop

Drop (telecommunication)

Drop, in telecommunication, has several meanings.In a communications network, a drop is the portion of a device directly connected to the internal station facilities, such as toward a telephone switchboard, toward a switching center, or toward a telephone exchange...

 impedance

Electrical impedance

Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current . Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases...

s, and prevents direct conduction

Electrical conduction

Electrical conduction is the movement of electrically charged particles through a transmission medium . The movement of charge constitutes an electric current. The charge transport may result as a response to an electric field, or as a result of a concentration gradient in carrier density, that is,...

 between the line and the drop.

It is a special application of an isolation transformer

Isolation transformer

An isolation transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits. An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block...

, and is often used to prevent ground loop

Ground loop (electricity)

In an electrical system, a ground loop usually refers to a current, generally unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...

s or earth loops, which cause humming or buzz

Buzz

A buzz can be a rapidly oscillating two-tone noise caused by vibration or other source, such as that produced by bees or wasps. It can also refer to a generally excited hubbub, or background noise — see for example marketing buzz....

ing in audio circuits

Electronic circuit

An electronic circuit is a closed path or paths formed by the interconnection of electronic components through which an electric current can flow....

.

In telecommunication

Telecommunication

Telecommunication is transmission over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, drums, semaphore, flags or heliograph. In modern times, telecommunication typically involves the use of electronic devices such as the telephone,...

s, a repeating coil is a voice-frequency 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...

 characterized by a closed magnetic core

Magnetic core

A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical and electromechanical devices such as electromagnets, transformers, electric motors, and inductors. It is usually made of ferromagnetic metal such as iron, or...

, a pair of identical balanced

Balanced line

In telecommunications and professional audio, a balanced line or balanced signal pair is a transmission line consisting of two conductors of the same type, each of which have equal impedances along their lengths and equal impedances to ground and to other circuits. The chief advantage of the...

 primary (line

Line (electrical engineering)

In electrical engineering, a line is, more generally, any circuit of an electrical system. This electric circuit loop , consists of electrical elements connected directly by conductor terminals to other devices in series.- Overview :Consumer electronic devices concerned with audio often have a...

) windings, a pair of identical but not necessarily balanced secondary (drop) windings, and low transmission loss

Attenuation

In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, and X-rays are attenuated by lead....

 at voice frequencies

Voice frequency

A voice frequency or voice band is one of the frequencies, within part of the audio range, that is used for the transmission of speech.In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz...

. It permits transfer

Transfer

Transfer may refer to:*Call transfer*Decal, a sticker*Electron transfer*Heat transfer*Jacoby transfer, a bidding device in contract bridge*Knowledge transfer...

 of voice currents from one winding to another by magnetic induction

Electromagnetic induction

Electromagnetic induction is the production of voltage across a conductor situated in a changing magnetic field or a conductor moving through a stationary magnetic field....

, matches line and drop

Drop (telecommunication)

Drop, in telecommunication, has several meanings.In a communications network, a drop is the portion of a device directly connected to the internal station facilities, such as toward a telephone switchboard, toward a switching center, or toward a telephone exchange...

 impedance

Electrical impedance

Electrical impedance, or simply impedance, describes a measure of opposition to a sinusoidal alternating current . Electrical impedance extends the concept of resistance to AC circuits, describing not only the relative amplitudes of the voltage and current, but also the relative phases...

s, and prevents direct conduction

Electrical conduction

Electrical conduction is the movement of electrically charged particles through a transmission medium . The movement of charge constitutes an electric current. The charge transport may result as a response to an electric field, or as a result of a concentration gradient in carrier density, that is,...

 between the line and the drop.

It is a special application of an isolation transformer

Isolation transformer

An isolation transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits. An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block...

, and is often used to prevent ground loop

Ground loop (electricity)

In an electrical system, a ground loop usually refers to a current, generally unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...

s or earth loops, which cause humming or buzz

Buzz

A buzz can be a rapidly oscillating two-tone noise caused by vibration or other source, such as that produced by bees or wasps. It can also refer to a generally excited hubbub, or background noise — see for example marketing buzz....

ing in audio circuits

Electronic circuit

An electronic circuit is a closed path or paths formed by the interconnection of electronic components through which an electric current can flow....

. It also prevents low direct current

Direct current

Direct current is the undirectional 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 be through...

 voltage

Voltage

Voltage is commonly used as a short name for electrical potential difference. Its corresponding SI unit is the volt...

s from passing.