Apparatus comprising: a radio frequency (RF) Robinson oscillator comprising: a resonator comprising a sensor rhumbatron, the sensor rhumbatron comprising a cavity member, the cavity member having a re-entrant boss member, the re-entrant boss member being arranged to project into a cavity portion of the cavity member; a feedback element arranged to provide positive radio frequency (RF) feedback to the cavity member thereby to increase a quality factor Q of the cavity member, the feedback element having first and second terminals coupled to the cavity member, the apparatus being operable to cause the oscillator to oscillate at a resonant frequency; and an output arranged to provide a signal that varies according to a value of at least one electrical parameter of the oscillator, said at least one electrical parameter being selected from amongst an electromagnetic loss and a resonant frequency.