the Hall effect
/ hawl /
The Hall effect is the sideways voltage that appears across a current-carrying strip of metal or semiconductor when it is placed in a magnetic field. Send a current down a flat ribbon, put it in a field pointing through the ribbon, and a tiny voltage springs up across its width, at right angles to both. It answers, can we detect and measure a magnetic field electrically, without any moving parts?
The mechanism is the Lorentz force on the moving charge carriers. As the carriers drift along the strip carrying the current, the magnetic field pushes them sideways, so charge piles up on one edge and is depleted on the other. This buildup creates a transverse electric field that grows until its push exactly balances the magnetic push; at that balance the carriers again flow straight. The resulting Hall voltage is V_H = I B / (n q t), where I is the current, B the field, t the thickness, n the number of carriers per unit volume, and q their charge. Measuring V_H thus reveals both the field strength and how many carriers there are.
The Hall effect matters for two big reasons. First, it is the basis of Hall-effect sensors, cheap and rugged devices that measure magnetic fields, sense position, and count wheel rotations in cars and phones. Second, the sign of the Hall voltage tells you the sign of the charge carriers, revealing that in some materials the current is effectively carried by positive 'holes' rather than negative electrons, a clue that helped build our understanding of semiconductors.
A Hall sensor in a car's crankshaft position gauge sits beside a spinning toothed wheel. Each tooth changes the nearby field, the Hall voltage pulses, and the engine computer counts the pulses to know exactly where the crankshaft is.
The sideways Hall voltage V_H = I B / (n q t) turns a magnetic field into a readable signal.
The Hall effect is one of the few simple ways to tell the sign of the charge carriers directly. The everyday statement 'current is a flow of electrons' hides the fact that in many materials the carriers act positive.