Hall effect
/ HAWL ih-FEKT /
Picture a crowd of people streaming down a wide hallway. Now imagine a steady wind blowing across the hall from one side. Everyone gets nudged toward the far wall and starts to pile up there. Electrons carrying a current behave the same way when a magnetic field blows across them — and that sideways pile-up of charge is the Hall effect.
Run a current lengthwise through a flat strip of conductor, then put a magnetic field at right angles to it, poking through the strip. The magnetic field pushes the moving charges sideways, so they crowd onto one edge and leave the other edge bare. That imbalance builds a small voltage across the strip — the Hall voltage — measured between the two edges, at right angles to both the current and the field. The size of this voltage tells you how densely packed the charge carriers are, and its sign reveals whether the current is carried by negative electrons or by positive holes.
The Hall effect matters because it is one of the few clean ways to look inside a material and count its charge carriers and tell their sign — a question you cannot answer from ordinary resistance alone. It is also the everyday workhorse behind position and current sensors in cars and phones. The common confusion is to expect the voltage along the direction of flow; the Hall voltage is the perpendicular one, and it vanishes the moment you switch off the magnetic field.
In 1879 Edwin Hall, then a graduate student, measured a faint sideways voltage across a thin gold leaf carrying current in a magnet — and in some metals the sign came out as if the current were carried by positive charges, a riddle only solved decades later by the idea of holes.
A single sideways voltage revealed that some conductors carry current with positive holes, not electrons.
At very low temperature and very strong magnetic field the ordinary Hall effect gives way to the quantum Hall effect, where the sideways response locks onto exact, perfectly reproducible values — a separate and far stranger phenomenon.