A classical charged particle with the mass M, the charge q and the angular momentum has a magnetic moment

The factor is called gyromagnetic ratio. If the particle is placed inside a magnetic field, it undergoes a torque:

The torque causes a change in the direction of the magnetic moment respectively the angular momentum. If the magnetic field is homogeneous and constant in time, the angular momentum will describe a circular orbit. This process is called precision. The frequency of the rotation yields:

This frequency is named after the British physician Larmor (1857-1942).

additional information:
Measurement of the Gyromagnetic Ratio