microtesla

The microtesla is the unit of the magnetic environment we actually live in. The Earth's field of 25 to 65 µT is the reference everything at this scale is compared against: a metre from most household appliances the mains-frequency field is well under 1 µT, and directly beneath a high-voltage transmission line it reaches a few microteslas. Anyone can read the unit directly, because every smartphone carries a three-axis magnetometer reporting in microteslas, and a free sensor app turns the phone into a µT meter that plainly shows the Earth's field, its dip angle, and the moment a steel beam or a speaker magnet comes near.

The gauss conversion trips people constantly at this scale: 1 µT is 10 milligauss, so the North American EMF surveys that quote milligauss give numbers ten times larger than the same field in microteslas.

The Earth's field is also not the steady backdrop it appears to be. It is weakening by very roughly 5 percent per century, and its weakest surface region, the South Atlantic Anomaly off the coast of South America, dips to about 22 µT. The dent has consequences: satellites passing through it see elevated radiation because the weakened field lets trapped particles reach lower altitudes, and instruments on Hubble and the International Space Station are managed around those passes. For the force side of the unit, the site's magnetic force solvers take a flux density, and entering 50 µT there shows how feeble the Earth's push on a current-carrying wire really is.