Let's take some atoms with a quantum spin (hydrogen for example). When placed in the magnetic field B₀ to be measured, these spins change orientation to find an equilibrium, corresponding to the lowest energy, when they are aligned with the field. The atoms then have to be stimulated with an electromagnetic wave: if the frequency corresponds exactly to their resonant frequency f₀, the spins will assume a different state (this is called the spin-flip). When the excitation process is stopped, the atoms relax, emitting a wave of measurable frequency f₀. Now, for a given type of atom, f₀ is a linear function of the ambient field B₀, where the gyromagnetic ratio y is constant for each material (y= 42.58 MHz/T for hydrogen).