What it shows:
When a magnetic field is applied perpendicular to a conductor carrying current, a potential difference is observed between points on opposite sides of the conductor. This happens because the magnetic field deflects the moving electrons (Lorentz force) to the edge of the conductor and the altered charge distribution generates a transverse electric field.
How it works:
The conductor is a small bar (11mm × 2mm × 2mm) of germanium (p-type?). Current (18 mA) is made to flow down the length of the bar by a 3 volt potential difference. Two pointed screws are used to pick up the transverse voltage across the bar. In the absence of a magnetic field, these points would be at the same potential, but due to the fact that they are not exactly opposite each other, there is a small (5½ mV) potential difference between them. Upon inserting the sample in a magnetic field (provided by a 2500 gauss "magnetron magnet"), the potential increases to +24½ mV. When the magnetic field is reversed (simply flip the sample over), the potential difference is -13½ mV. The change in potential is 38 mV and we deduce the Hall Voltage to be half of that, or 19 mV.
Setting it up:
The germanium sample and digital voltmeter can be shown on a document camera. The current is provided by two 1½ volt batteries in series.
The effect was discovered in 1879 by E.H. Hall. In those days no one understood the mechanism of conduction in metals. A complete understanding of the Hall effect came only with the quantum theory of metals, about 50 years after Hall's discovery.