What is the unit of magnetic flux density?

The tesla, and it is a very large unit - the Earth's magnetic field is about 50 microtesla, a fridge magnet is around 5 millitesla, and an MRI scanner runs at 1.5 or 3 tesla, which is tens of thousands of times the planet's field.

That is why MRI rooms are dangerous in a specific way. The magnet is always on, and anything ferromagnetic brought into the room becomes a projectile - oxygen cylinders, floor polishers, chairs and mop buckets have all been pulled in, and a child was killed in 2001 when an oxygen tank was carried in during a scan. The screening protocols are strict because the failure mode is instantaneous.

Nikola Tesla, who won the alternating current argument against Edison and gave the world the induction motor, died in a New York hotel room in 1943 owing money, having spent his last years feeding pigeons. Edison won the public reputation for decades and Tesla's has since overtaken it, partly on the strength of the AC system that everything now runs on and partly, it must be said, on the internet finding him a satisfying figure.