Induction loops use Faraday’s law to detect changes in magnetic fields caused by ferrous metals, such as cars, to activate traffic signals. They are also used to improve hearing aid effectiveness in rooms and taxis. Highly conductive loop wire materials, such as iron and copper, amplify the magnetic field and detect smaller metal masses, such as bicycles. Induction loops are considered more practical and cost-effective than other traffic control methods.
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. A common example where an induction loop is used as of 2011 is in buried electrical cables at automobile traffic intersections. A constant current passing through the field will be disturbed when a ferrous metal such as steel approaches it and this can be used to activate traffic signal controls. Induction loops are also becoming increasingly popular to meet the needs of people who are hard of hearing and rely on hearing aids.
The steel frame of a car as well as lighter vehicles such as bicycles will disturb the current flow in an induction loop as they travel to an intersection. Such intersections are equipped with circuits that can sense how many vehicles are in a line and adjust the frequency of the signal to improve traffic flow. While this is not the only way to automatically control traffic at intersections, it is considered to be more practical and cost-effective 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 loop wire materials such as iron, steel, and copper, and by placing inductance wires stacked on top of each other in a series of slightly off-center wire circles or rectangles . This is useful for detecting bicycles or motorcycles as they have a much smaller total metal mass than cars or trucks. 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 serves as the core extended for the wiring itself. This magnetic amplification prevents the flow of electric current in the circuit as it acts as a sort of inductance cut off on the standard alternating current (AC) being used. Such changes are monitored by control loops to record how many cars are present, or an 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 as 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 are held, known as an audio frequency induction loop (AFIL). They can also be placed around the inside perimeter of cars and, in the UK, are a requirement in all public taxis. Hearing aid technology that draws on the induction loop to amplify sounds was originally thought to capture the magnetic field generated by a telephone to amplify the sound signal, and was known as the telephone switch or telecoil on the hearing aid.
When someone speaks into a microphone in such a room or as a taxi driver, the induction loop shows corresponding changes in its magnetic field, which the hearing aid detects and translates into sound. This is important for the deaf, as hearing aids are often ineffective at transmitting accurate sounds over a distance to the user. As a sound wave becomes more and more distant, its higher-frequency elements that make speech intelligible fade away, as does the overall loudness. This, along with the mixed in background distortion noise, are elements of sound for which hearing aids cannot compensate, and an induction loop in a room negates these effects simultaneously for everyone who is using a hearing aid.
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