Abstract: An apparatus for monitoring the inductance of a coil (L1) which is not part of a resonant circuit including a second coil (L2) connected in series with the first mentioned coil and a resonant circuit which is formed by connection of a capacitor (C5) to the second coil, or optionally (C2) to a third coil (L3) which is in a mutually inductive relationship with the second coil so that the second and third coils act as the primary and secondary coils of a transformer (M), a device (1) for injecting into the circuit containing the first mentioned and second coils an a.c. signal of substantially constant peak voltage amplitude having a frequency within the resonance peak of the resonant circuit, and an amplitude detector (5) for measuring the amplitude of the voltage across a component of the resonant circuit. The apparatus may be employed as so-called integral control in closed loop control of the suspension of an object by the first mentioned coil.