Electron spin resonance spectrometer

Electron spin resonance spectrometers comprise a sample cavity located between the poles of a permanent magnet, means for applying microwave energy to the cavity, means for measuring the frequency of the microwave energy applied to the cavity, means for controlling the magnetic field strength in the cavity, means for detecting changes in cavity Q and accordingly changes in the level of microwave power reflected back from the cavity, and means for analysing the changes in reflected power. In accordance with one aspect, a nuclear magnetic resonance probe including a standard sample is placed adjacent the sample cavity, means are provided for measuring the nuclear magnetic resonance frequency in the standard sample, calculating the magnetic field to which the sample and the adjacent cavity are subjected therefrom and applying the results of the calculation to control the magnetic field strength in the cavity. In another aspect the spectrometer includes control means for controlling the operation of the spectrometer by stabilising the precisely defined relationship between magnetic field strength and microwave frequency at resonance in accordance with a fixed or swept demand g-value relating to the test sample.