Abstract: The apparatus can be used for detecting a liquid contaminant in a rotating cavity of an electric machine. The apparatus can have a magnetic field source mounted to a non-rotary base and transmitting a magnetic field across a gap between the base a housing of the cavity, an interrogation circuit having a coil magnetically coupled to the magnetic field source across the gap via the magnetic field, and a distal circuit portion having electrical contacts separated by a spacing fluidly communicating with the cavity; and a detection circuit mounted to the base and having a coil magnetically coupled to the magnetic field and a detector detecting a variation occurring in the detection circuit upon change of a permittivity of the liquid across the spacing, the variation being indicative of the liquid contaminant.