tesla
Magnetic flux densityexact by definition
The tesla is the SI unit of magnetic flux density, defined as one weber of magnetic flux per square metre. Equivalently, a field of one tesla exerts one newton of force per ampere metre on a conductor at right angles to it. As a coherent derived SI unit its relation to the base units is exact by definition.
Watch out: The tesla measures magnetic flux density, written B, not magnetic field strength, written H and expressed in amperes per metre. The two differ by the permeability of the material, which for iron is thousands of times that of free space, so the distinction matters as soon as a magnetic core is involved.
| 1 T | 1 T |
Named for Nikola Tesla, whose polyphase alternating-current machines made the rotating magnetic field the basis of industrial power. The unit was adopted by the CGPM in 1960.
About the tesla
One tesla is a strong field. The Earth manages about 50 microteslas, a refrigerator magnet a few milliteslas at its face, and a good neodymium magnet perhaps half a tesla at the pole. Clinical MRI scanners run at 1.5 or 3 T, research scanners at 7 T, and the strongest continuous laboratory magnets reach the mid-40s in tesla with pulsed systems going far higher. The scale is why the tesla replaced the gauss slowly: for most everyday magnetism the gauss, at \(10^{-4}\) T, gives friendlier numbers.
Flux density is what appears in the two force laws that matter. A moving charge feels \(F = qvB\sin\ heta\), and a current-carrying wire feels \(F = BIL\sin\ heta\). Both are perpendicular to the motion and to the field, which is why motors turn rather than push and why charged particles in a field travel in circles. The same \(B\) drives induction through \(\mathcal{E} = -d\Phi/dt\), so it is simultaneously the quantity that makes motors work and the quantity that makes generators work.