SI coherent derived unit with special name and symbol
Name Symbol Derived quantity Expressed in terms of SI base units
tesla T magnetic
flux density
kg s−2 A-1


The tesla, symbol T, is the SI coherent derived unit of magnetic flux density, and of magnetic field strength.

The tesla is named after the Serbian-American electrical engineer Nikola Tesla.

Magnetic fields

Magnetic fields surround, and are produced by, magnetised material and electric currents (moving electric charges). They exert forces on moving electric charges, and torques on magnetic dipoles.

Both the strength and direction of a magnetic field vary with location. As such, a magnetic field is described mathematically as a vector field.

The term “magnetic field” is used for two distinct but closely related fields:

  • magnetic field strength, symbol H, measured in amperes per metre, symbol A m-1
  • magnetic flux density, symbol B, measured in tesla, symbol T

In vacuum, B and H are the same apart from the units used, but in a magnetised material, B and H differ by the magnetisation M of the material at that point in the material. H is the applied magnetising field strength, and B is the resulting induced magnetic flux.

Magnetic fields and electric fields are both components of the electromagnetic force.

Lorentz force

A particle, carrying a charge of one coulomb, and moving perpendicularly through a magnetic field of one tesla, at a speed of one metre per second, experiences a force of magnitude one newton.


The finest precision for a magnetic field measurement was attained by Gravity Probe B at 5 aT (5×10−18 T).

Magnetometers are devices used to measure the local magnetic field.

Important classes of magnetometers include using induction magnetometer (or search-coil magnetometer) which measure only varying magnetic fields, rotating coil magnetometer, Hall effect magnetometers, NMR magnetometers, SQUID magnetometers, and fluxgate magnetometers.

The magnetic fields of distant astronomical objects are measured through their effects on local charged particles. For instance, electrons spiraling around a field line produce synchrotron radiation that is detectable in radio waves.