It takes a 16-tesla magnet to make a living frog float
A fridge magnet manages about 5 millitesla. A hospital MRI scanner works at 1.5 to 3 tesla. Push to 16 tesla and the water in a frog's body is repelled strongly enough to levitate the animal, a feat that earned an Ig Nobel Prize in 2000. The unit behind these numbers honours Nikola Tesla.
The tesla measures magnetic flux density, the B-field, in the International System of Units. It was announced at the General Conference on Weights and Measures in 1960, at the suggestion of the Slovenian electrical engineer France Avcin, in honour of the Serbian-American engineer. As with other units named after people, the symbol is a capital T while the name is written in lowercase.
One tesla is one weber of magnetic flux spread over a square metre. A more physical picture comes from the Lorentz force: a charge of one coulomb moving at one metre per second at right angles through a one-tesla field feels a push of one newton. The older CGS unit, the gauss, is ten thousand times smaller, but the two do not mean quite the same thing and NIST does not accept gauss alongside SI units. Geophysicists once used the gamma, now replaced by the nanotesla. The 2019 overhaul of the SI, which redefined the ampere, shifted the tesla by about 107 parts per billion and made tesla-to-gauss conversions slightly uncertain, so journals now discourage them.
Earth's own field at the surface is tiny, between 25,000 and 65,000 nanotesla, and standing beneath a high-voltage power line exposes you to around 40 microtesla. The superconducting magnet around CERN's CMS detector reaches 4 tesla, and a magnet developed by MIT and Commonwealth Fusion Systems for fusion reactors runs at 20.
Laboratory peaks climb steeply. Continuous-field magnets reached 45 tesla in 2015, and the strongest non-destructive magnet has hit 97.4. A flux-compression technique produced about 1,200 tesla for roughly 100 microseconds. Nature goes further: a typical white dwarf star carries about 100 tesla, and above roughly a billion tesla, the Schwinger limit, the electromagnetic field itself should stop behaving linearly.
Source: Tesla (unit)
