* Larmor precession* is the change in orientation of the axis of the magnetic moment of a particle with respect to the axis of an external magnetic field. Consider an electron at rest with intrinsic spin of in a -directional uniform magnetic field . The magnetic field interacts with the electron’s magnetic moment and generates a torque (see diagram below).

If is the spin magnetic moment of an electron, eq61 becomes , where is the * gyromagnetic ratio* of the electron. Taking the derivative on both sides of this equation with respect to ,

From eq64 and eq71, we have , whose spin analogue is . So,

With reference to the above diagram, the change in arc length with respect to is . Hence,

where is the period, is the mass of an electron and is the * Larmor frequency*, which is defined as a positive value.

Note that the Larmor frequency is sometimes defined as , which is then .