In telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

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, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, inductors and magnetic assemblies. It is made of ferromagnetic metal such...

, 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....

) 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, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...

 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:* Transfer * Transfer * Transfer DNA, the transferred DNA of the tumor-inducing plasmid of some species of bacteria such as Agrobacterium tumefaciens* Transfer...

 of voice currents from one winding to another by magnetic induction

Electromagnetic induction

Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

, 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. A drop can also be a wire or...

 impedance

Electrical impedance

Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

s, and prevents direct conduction between the line and the drop.

It is a special application of an isolation transformer

Isolation transformer

An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...

, and is often used to prevent ground loop

Ground loop (electricity)

In an electrical system, a ground loop usually refers to a current, almost always 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

-People:*Buzz Aldrin , American pilot and astronaut, second person to set foot on the Moon*George Beurling , Canadian World War II fighter pilot*Buzz Hargrove , National President of the Canadian Auto Workers trade union...

ing in audio circuits

Electronic circuit

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...

. It also prevents low direct current

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...

 voltage

Voltage

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 from passing.