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The induction principle applies to a hearing aid when the hearing aid has a feature called a "Telecoil". A Telecoil is simply a very small coil that serves as one coil in the transformer analogy. It serves as an input source on a hearing aid the same as a microphone except it gets it's signals electromagnetically rather than acoustically.  The signal from a Telecoil is very weak so it has to be amplified by the hearing aid. Other devices, like a telephone handset, also contain a coil of wire. When the telephone handset is placed close to the hearing aid, the two coils are close together and induction can take place.

When the two coils are close together, maximum power transfer takes place, and when they are moved away, power transfer drops off. At one time, the Telecoil was thought to be only useful with a telephone, and the switch that activates a Telecoil was called the "Telephone Switch", or "T" Switch.

It is still called a "T" switch, but today the Telecoil is used with several other things besides a telephone. However, less than half of the hearing aids in use today have the Telecoil feature. That may be because (1) Hearing aid dispensers do not realize the value of telecoils and (2) many hearing aids are too small to contain a telecoil. Those of us who do value our telecoils are "In the Loop".        
The Telecoil is used with:
Telephones. Starting in 1990, the HAC (Hearing Aid Compatibility Act) required that all phones, except cell phones, must contain a coil that will induce a speech signal in the Telecoil inside a hearing aid.
Neckloops. A neckloop is a coil of wire that is worn like a necklace. It plugs into other devices such as radio receivers, tape players, Infrared Receivers, and TVs that have a headphone jack. Electrically, the neckloop is similar to headphones so most things that will accept headphones will also accept neckloops.
Silhouettes. A silhouette is a coil of wire that is imbedded in plastic and is shaped like a hearing aid. It has a hook to help hold it on the ear and it is placed right next to the hearing aid. Since it is very close to the hearing aid, induction takes place much more efficiently, which makes it more sensitive than a neckloop and can be used when a neckloop is too weak. Silhouettes plug into devices the same as neckloops and headphones.
Room Size Audio Loops. The audio loop is a big coil of wire that goes all around the circumference of a room.  An Audio loop may be just big enough to provide coverage for a room, or it may cover an entire house or apartment, or it may be auditorium sized.  The Audio loop is the main subject of these pages.  

All of the above allows a hearing aid user to make maximum use of Hearing Assistive Technology (HAT). This technology includes Radio systems - usually FM, Infrared Systems, and the subject of these pages - the AudioLoop. HAT allows a person to hear better because sound is transmitted electronically from the source, rather than acoustically. It has the effect of being close to the sound source so environmental distortions don't take place.