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The tesla is an SI unit for measuring the strength of a magnetic field. Its symbol is T. The unit is named after the famous inventor Nikola Tesla, who discovered the alternating current, among hundreds of other phenomena. A magnetic field of one tesla is about 30,000 times as powerful as the Earth's magnetic field. However, because the quantity of teslas is computed by dividing the total magnetic flux (power) by area, magnetic fields of high tesla can be achieved by concentrating them in a small space.
For a magnetic flux density to equal 1 tesla, a force of 1 newton must act on a wire of length 1 meter carrying 1 ampere of current. A newton -- the force required to accelerate a 1 kg weight at one meter per second squared -- is a lot of force for a magnetic field to exert, and is not easily achieved. The most powerful superconducting electromagnets only produce magnetic fields of around 20T.
The most powerful continuous magnetic field yet generated measures 45T, and the strongest destructive pulse magnet about 850T. A gauss, another unit for measuring magnetism, is 1/10,000th of a tesla. The gamma, still another unit used in geophyics to measure magnetic fields, is one billionth of a tesla.
A weber, another SI unit, is used to measure magnetic flux, whereas the tesla is used to measure magnetic flux density, commonly understood as a magnetic field. It is possible for a given material to become entirely saturated with magnetic flux. For example, 10 teslas is considered to be the upper limit of niobium-titanium accelerator magnets.
Medical magnetic resonance imaging typically sustains a field strength of 2T. A large loudspeaker magnet generates 1T. Exotic cosmic objects such as the magnetar, a neutron star with a massive magnetic field, produce between 0.1 and 100 gigateslas. This is enough to wipe a credit card at a distance equivalent to the gap between the Earth and the Sun.