Ampere-turn or AT is the SI unit of magneto-motive force and is a direct measure of a reed contact’s magnetic sensitivity.
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The ampere-turn (At) is the MKS (Metres, Kilograms, Seconds) unit of magnetomotive force (MMF), represented by a direct current of one ampere flowing in a single-turn loop in a vacuum. "Turns" refers to the winding number of an electrical conductor composing an inductor.
For example, a current of 2 A flowing through a coil of 10 turns produces an MMF of 20 At.
By maintaining the same current and increasing the number of loops or turns of the coil, the strength of the magnetic field increases because each loop or turn of the coil sets up its own magnetic field. The magnetic field unites with the fields of the other loops to produce the field around the entire coil, making the total magnetic field stronger.
The strength of the magnetic field is not linearly related to the ampere-turns when a magnetic material is used as a part of the system. Also, the material within the magnet carrying the magnetic flux "saturates" at some point, after which adding more ampere-turns has little effect.
Alternatively, NI (the product of the number of turns, N, and the current [in amperes], I) has been used in industry, specifically, US-based coil-making industries.