Electrical currents are affected by magnetic fields. When a magnetic field is applied perpendicular to the flow of current, the field causes resistance in the current. This is a manifestation of the Lorentz force, which pushes the negatively charged electrons in the current in a direction dictated by the left hand rule. This movement of electrons results in a weak but measurable potential difference, or voltage, perpendicular both to the current flow and the applied magnetic field. This is known as the Hall effect, named after American physicist Edwin Hall, who discovered the phenomenon in 1879. This effect is particularly pronounced in thin metals, and is easily observable in a low-density plasma (an electrically conductive ionized gas), such as a fluorescent light, as in this tutorial.
The ends of the Hall Effect Tube (a vacuum with a trace of neon gas) are connected to a Battery. A smaller tube intersects with the larger tube; attached to its ends are electrodes that are connected to a Voltmeter to detect voltage. Notice that the neon in the tube is fluorescing (emitting light) due to the excitation of the electrical current supplied from the battery. This current is reflected in the flow of electrons, depicted here as small yellow particles. Adjust the Magnet Position slider to move the strong permanent magnet closer to the Hall Effect Tube. Check the voltmeter; the Hall effect causes a Hall voltage perpendicular to the main current. Observe how the flow of electrons responds to the magnetic field. Experiment with the blue Flip Magnet and Flip Battery buttons to see how this affects the potential difference. Clicking on the Field Lines box will show the invisible magnetic forces at work and help you to visualize this.
A similar effect is seen in semiconductors, where the Hall effect plays a large role in the design of integrated circuits on semiconductor chips. In most conductors, such as metals, the Hall effect is very small because the density of conduction in electrons is very large and the drift speed (charged particle erraticism) is extremely small, even for the highest obtainable current densities. The Hall effect is therefore considered unimportant in most electric circuits and devices and is not mentioned in many texts on electricity and magnetism. However, in semiconductors and in most laboratory plasmas, the current density is many orders of magnitude smaller than in metals, and the Hall effect is correspondingly larger and is often easily observable. Some devices for measuring magnetic fields make use of semiconductors as the sensing elements and are called Hall probes.