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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.[1] "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 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, when adding more ampere turns has little effect.

The ampere-turn is equal to gilberts, the equivalent CGS unit.

Alternatively, NI has been used in industry, specifically, US based coil making industries.

In Thomas Edison’s laboratory Francis Upton was the lead mathematician. Trained with Helmholtz in Germany, the unit of current was referred to as a Weber, modified to Ampere later:

When conducting his investigations, Upton always noted the Weber turns and with his other data had all that was necessary to put the results of his work in proper form. ¶ He discovered that a Weber turn (that is, an ampere turn) was a constant factor, a given number of which always produced the same effect magnetically.[2]

See also[edit]


  1. ^ "ampere–turn". Merriam-Webster. Retrieved 6 November 2012. 
  2. ^ Francis Jehl (1937) Menlo Park Reminiscences, page 310, Edison Institute