What Is an Induction Loop?

The induction loop is a form of technology used for a variety of purposes that is based on Michael Faraday’s discovery of the law of induction, also known as Faraday’s law, in 1831. The principle is based on the dual magnetic and electrical properties of electromagnetic fields that are generated in electrical circuits. One common example where an induction loop is used as of 2011 is in buried electrical cables at automobile traffic intersections. A steady current passing through the field will be disturbed when a ferrous metal such as steel comes within close proximity to it and this can be used to trigger traffic signal controls. Induction loops are also becoming increasingly widespread to accommodate the needs of individuals who are hard of hearing and rely on hearing aid devices.

The steel frame of a car as well as lighter vehicles like bicycles will disturb the flow of current in an induction loop as they move up to an intersection crossing. Such intersections are provided with loops that can detect how many vehicles are in a line, and adjust signal frequency to improve traffic flow. While this is not the only way of controlling traffic automatically at intersections, it is considered more practical and inexpensive than methods that use cameras or heat-based infrared sensors mounted on the signal lights themselves to detect vehicles.

The power of traffic-control induction loops is amplified by using highly conductive materials for the loop wires such as iron, steel, and copper, and by placing overlapping inductance cables on top of each other in a series of slightly off-center wire circles or rectangles. This is useful for detecting bicycles or motorcycles since they have a much smaller total metal mass than cars or trucks do. The frequency of such loops is usually in the range of 20,000 to 30,000 hertz, and, when a car or other conducting object passes over them, the magnetic field present in the induction loop is amplified by the additional metal, which acts as an extended core for the wiring itself. This magnetic amplification impedes the flow of electrical current in the loop as it acts as a sort of inductance break on the standard alternating current (AC) being used. Such changes are monitored by control circuits to register how many cars are present, or a general overall metal mass level at each point in a traffic light crossing, so that the lights can be changed accordingly.

Another common application as of 2011 for the induction loop principle is in that of a room-based device to increase the effectiveness of hearing aids. A loop of wire is typically run around the perimeter of a room where lectures or other gatherings take place that is known as an audio-frequency induction loop (AFIL). They can also be placed around the interior perimeter of cars, and, in the UK, they are a requirement in all public taxis. The technology in hearing aids that taps into the induction loop to amplify sounds was originally meant to capture the magnetic field generated by a telephone to amplify the sound signal, and was known as a telephone or telecoil switch on the hearing aid.

As someone speaks into a microphone in such a room or as a taxi driver, the induction loop displays corresponding alterations in its magnetic field, which the hearing aid picks up and translates into sound. This is important for the hard of hearing, as hearing aids are often ineffective at carrying accurate sounds from a distance to the user. As a sound wave becomes increasingly distant, its higher frequency elements which make it understandable speech fade away, as does the overall volume. This, along with background distortion noise mixed in, are elements of sound for which hearing aids cannot compensate, and an induction loop in a room negates these effects simultaneously for everyone in the room who is using a hearing aid.