Abstract: A method of initiating and monitoring partial discharges in an insulating medium is disclosed. Apparatus for performing the method is also disclosed. A relatively low frequency time varying exciting waveform is applied to a sample of an insulating medium. A trigger pulse of short duration is superimposed on the exciting waveform to initiate partial discharge activity in the sample. The apparatus includes a trigger pulse generator with a synchronizing circuit which provides a pre trigger signal for an oscilloscope, allowing the resultant partial discharge activity to be monitored.