A device to maintain automatically the frequency of any source of alternating voltage within specified limits. The device is ‘error-operated’ and usually consists of two parts: one part is a frequency discriminator that compares the actual and desired frequencies and produces a d.c. output voltage proportional to the difference between them, with sign determined by the direction of drift; the other part is a reactor that forms part of the oscillator tuned circuit and is controlled by the discriminator output in such a way as to correct the frequency error.