Hallway

Meet Hallway, a strip of conductor who builds a sideways voltage whenever a magnetic field pushes on her current.

Charges moving through a conductor in a magnetic field feel a sideways magnetic force. They drift to one edge until the electric field from the piled-up charge balances the magnetic push. The voltage across the strip is the Hall voltage, discovered by Edwin Hall in 1879. It is IB/(nqt): it grows with current and field, and is smaller when there are more carriers to share the current.

The sign of the Hall voltage reveals the sign of the carriers. In most metals they are electrons, but in some semiconductors the current behaves as if it is carried by positive 'holes'.

Hall sensors are cheap and everywhere: they measure magnetic fields, detect wheel and shaft rotation in cars, and sense the lid closing on many laptops.

Metals have huge carrier densities, so their Hall voltages are tiny. Semiconductors, with far fewer carriers, give much larger signals, which is why sensors use them.