Abstract: A method and apparatus of monitoring the resistance characteristics of a circuit carrying an alternating current or a current having an alternating component is described. The apparatus includes first integration or averaging means for integrating or averaging the potential drop for the period between instants when the instantaneous current has the same value in a current loop; and display means for displaying a value representative of the output from the integration or averaging means.

Abstract: Power supply apparatus is disclosed including an inverter, such as a half-bridge inverter, and control means for controlling the frequency of operation of the inverter. The control means includes a further control winding on the transformer of the invertor, and means for applying a short circuit of one or other or both polarities to the control winding in order to activate a switching transition of the inverter. In one embodiment, the control means is used to fix the frequency of operation of a free-running inverter; in another embodiment, short circuits of both polarities are applied to the control winding for an overlap period in order to provide a "dwell" period between switch-off of one inverter transistor and switch-on of the other, this method of operation providing pulse width modulation control of the output waveform of the inverter.