The **tesla** (symbol **T**) is a unit of measurement of the strength of a magnetic field. It is a derived unit of the International System of Units, the modern form of the metric system.

One tesla is equal to one weber per square metre. The unit was announced during the General Conference on Weights and Measures in 1960 and is named in honour of Nikola Tesla, upon the proposal of the Slovenian electrical engineer France Avčin.

The strongest fields encountered from permanent magnets are from Halbach spheres and can be over 4.5 T. The strongest field trapped in a laboratory superconductor as of June 2014 is 21 T. The record magnetic field has been produced by scientists at the Los Alamos National Laboratory campus of the National High Magnetic Field Laboratory, the world's first 100-tesla non-destructive magnetic field.

A particle, carrying a charge of one coulomb, and passing through a magnetic field of one tesla, at a speed of one metre per second, perpendicular to said field, experiences a force with magnitude one newton, according to the Lorentz force law. As an SI derived unit, the tesla can also be expressed as

(The last equivalent is in SI base units).

Units used:

- A = ampere
- C = coulomb
- kg = kilogram
- m = metre
- N = newton
- s = second
- H = henry
- V = volt
- J = joule
- Wb = weber

In the production of the Lorentz force, the difference between these fields is that a force from a magnetic field on a charged particle is generally due to the charged particle's movement, while the force imparted by an electric field on a charged particle is not due to the charged particle's movement. This may be appreciated by looking at the units for each. The unit of electric field in the MKS system of units is newtons per coulomb, N/C, while the magnetic field (in teslas) can be written as N/(C·m/s). The dividing factor between the two types of field is metres per second (m/s), which is velocity. This relationship immediately highlights the fact that whether a static electromagnetic field is seen as purely magnetic, or purely electric, or some combination of these, is dependent upon one's reference frame (that is, one's velocity relative to the field).

In ferromagnets, the movement creating the magnetic field is the electron spin (and to a lesser extent electron orbital angular momentum). In a current-carrying wire (electromagnets) the movement is due to electrons moving through the wire (whether the wire is straight or circular).

One tesla is equivalent to:

- 10,000 (or 10
^{4}) G (gauss), used in the CGS system. Thus, 10 kG = 1 T (tesla), and 1 G = 10^{−4}T. - 1,000,000,000 (or 10
^{9}) γ (gamma), used in geophysics. Thus, 1 γ = 1 nT (nanotesla). - 42.6 MHz of the
^{1}H nucleus frequency, in NMR. Thus, the magnetic field associated with NMR at 1 GHz is 23.5 T.

One tesla is equal to 1 V·s/m^{2}. This can be shown by starting with the speed of light in vacuum, *c* = (ε_{0}μ_{0})^{−1/2}, and inserting the SI values and units for *c* (×10^{8} m/s), the 2.998vacuum permittivity ε_{0} (×10^{−12} A·s/(V·m)), and the 8.85vacuum permeability μ_{0} (×10^{−7} T·m/A). Cancellation of numbers and units then produces this relation. 12.566

For those concerned with low-frequency electromagnetic radiation in the home, the following conversions are needed most:

- 1000 nT (nanotesla) = 1 µT (microtesla) = 10 mG (milligauss),
- 1,000,000 µT = 1 T.

For the relation to the units of the magnetising field (ampere per metre or oersted), see the article on permeability.

- 31.869 µT (3.2 × 10
^{−5}T) – strength of Earth's magnetic field at 0° latitude, 0° longitude - 5 mT – the strength of a typical refrigerator magnet
- 0.3 T – the strength of solar sunspots
- 1.25 T – magnetic flux density at the surface of a neodymium magnet
- 1 T to 2.4 T – coil gap of a typical loudspeaker magnet
- 1.5 T to 3 T – strength of medical magnetic resonance imaging systems in practice, experimentally up to 17 T
- 4 T – strength of the superconducting magnet built around the CMS detector at CERN
- 8 T – the strength of LHC magnets
- 11.75 T – the strength of INUMAC magnets, largest MRI scanner
- 13 T – strength of the superconducting ITER magnet system
- 16 T – magnetic field strength required to levitate a frog (by diamagnetic levitation of the water in its body tissues) according to the 2000 Ig Nobel Prize in Physics
- 17.6 T – strongest field trapped in superconductor in lab as of July 2014
- 27 T – maximal field strengths of superconducting electromagnets at cryogenic temperatures
- 35.4 T – the current (2009) world record for a superconducting electromagnet in a background magnetic field
- 45 T – the current (2015) world record for continuous field magnets
- 100 T – The magnetic field strength of the average White dwarf star
- 10
^{8}– 10^{11}T (100 MT – 100 GT) – magnetic strength of the average magnetar

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