Device for the frequency measurement, and / or frequency control, in particular for low frequency alternating current, in which the voltage of the frequency to be measured by means of a weakly damped, in the vicinity of the resonance point resonant circuit of the operating primaerwicklung of a transformer is applied, the two sekundaerwicklungenin of such circuit has one end of each sekundaerwicklung with the grid, the other end with the cathode is connected in each case of a pipe