The ampere-turn (At) is the SI 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 comprising an inductor.

For example, a current of 2A flowing through a coil of 10 turns produces an MMF of 20At.

Here we see that by maintaining the same current and simply increasing the number of loops or turns of the coil, the strength of the magnetic field increases. This is because each loop or turn of the coil sets up its own magnetic field, which unites with the fields of the other loops to produce the field around the entire coil. The more loops, the more magnetic fields unite and reinforce each other and, as a result, the total magnetic field becomes stronger.

The strength of the magnetic field is not linearly related to the ampere turns. Also the material within the magnet carrying the magnetic flux "saturates" at some point, when adding more ampere turns has little effect.

The ampere-turn is equal to 4π / 10 gilberts, the equivalent CGS unit.

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