ampere per metre

ampere per metre

SI coherent derived unit
Name Symbol Derived quantity
ampere per metre m−1 A magnetic field strength


The ampere per metre, symbol m−1 A, is the SI coherent derived unit of magnetic field strength.

Magnetic fields

A magnetic field is a vector field that describes the magnetic influence of electrical currents and magnetised materials. In everyday life, the effects of magnetic fields are often seen in permanent magnets, which pull on magnetic materials (such as iron) and attract or repel other magnets.

Magnetic fields surround and are created by magnetised material and by moving electric charges (electric currents) such as those used in electromagnets.

Magnetic fields exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field varies with location. As such, it is an example of a vector field.

The term ‘magnetic field’ is used for two distinct but closely related fields denoted by the symbols B and H:

  • H is measured in units of amperes per metre, and
  • B is measured in teslas, which are equivalent to newtons per metre per ampere.

H and B differ in how they account for magnetisation. In vacuum, B and H are the same aside from units; but in a magnetised material, B0 and H differ by the magnetisation of the material at that point in the material.