A method and apparatus for measuring reactance of a sensor, such as a coil 4 used for e.g. eddy current proximity measurements, for example between blade tips and casing in a jet engine or other turbo-machine, by measuring the phase of a signal dependent on the reactance of the sensing component, possibly a capacitor. Source 2 drives alternating current, possibly via a unit having fixed reactance 6A, 6B (possibly inductors 16, 18 & capacitances 20, 22) through the sensor 4. A measure of reactance is output 10 based upon a phase measurement possibly using a phase-locked loop (PLL) situated at output unit 30, which converts the phase difference between signals 41, 42 into an analogue voltage proportional to the measured phase. A temperature measurement e.g. by monitoring the resistance of the sensor 4 if a coil, may provide for a temperature corrected measure of reactance.
