Hall probe

A Hall probe is a measuring device for determining the strength of magnetic fields. The magnetic field itself is measured in A/m (amperes per metre) or oersted. In science and technology, however, people are more accustomed to the tesla unit of measurement, which is used for magnetic flux density. This is why Hall probes usually display the value of the magnetic flux density in tesla. One could also build a Hall probe that directly indicates the value for the force on a specific piece of iron in a magnetic field.

Using the Hall effect for Hall probes

The Hall probe utilises the Hall effect to determine the magnetic flux density. Due to the Lorentz force, a force that is perpendicular to the direction of charge carrier motion acts on charge carriers moving in a magnetic field. This drives them towards one side of the charge carrier. The Hall effect was discovered by the American physicist Edwin Hall in 1879.

The Lorentz force is a force that always acts on moving charges in magnetic fields. If a current is applied to a metal plate in a magnetic field, a force acts on the carriers of the current, i.e. the electrons.

The direction of the force is perpendicular to the direction of movement of the electrons and perpendicular to the magnetic field. The formula for the Lorentz force \(\vec{F}\) on charge carriers of velocity \(\vec{v}\) in the magnetic flux density \(\vec{B}\) is \(\vec{F}=q\vec{v}{\times{\vec{B}}}\), where q denotes the charge. Therefore, \(\vec{F}=-e\vec{v}{\times{\vec{B}}}\) applies to electrons, as the charge of the electron is a negative elementary charge e. An oppositely directed force would act on positive charges. The Hall effect can thus also be used to determine that the particles that are in motion when the current flows (i.e. the electrons) carry a negative charge and not a positive one.

In the current-carrying plate of the Hall probe, the electrons are now displaced vertically from their direction of movement and collect towards one side of the plate. This results in an electrical voltage across the width of the plate, which is proportional to the magnetic field to be measured. From the value of the so-called Hall voltage U across the plate, the external magnetic field in which the plate is located can then be specified with the help of a further conversion of the simultaneously acting electrical forces that are in equilibrium with the Lorentz force.