Hall Effect
Definition
How we measure magnetic fields with a chip — current bent sideways by a magnet builds up a measurable voltage.
Technical definition
V_H = IB/nqd. Charge carriers deflected by the Lorentz force create a transverse voltage whose sign reveals whether carriers are electrons or holes.
Example
Your phone's compass is a Hall sensor — it measures Earth's ~50 μT field to determine which way is north.
Common confusion
The Hall voltage is perpendicular to both the current and the field. Drawing it along the wire misses the point — the sideways magnetic push is the whole effect.