Classifying DC/ULF electric field sensors by their Thevenin representation

There exist several methods for sensing electric fields at the lowest end of the electromagnetic spectrum. The best known method perhaps is the one related to sensors whose sensing electrodes are in steady movement. The operation of DC field sensors is due to the movement of their sensing electrodes, which enables coupling of the sensor to the field sources in spite of the fact that the sources are DC ones. We show that the operation of most of the DC sensors is made explicable by appreciating that their electrodes movement creates a time varying coupling capacitance in their Thevenin equivalent. It is further shown that the latter Thevenin circuit, which possesses a DC source, can be converted to another Thevenin equivalent with an AC source. It enables a much simpler evaluation. The electrode movement in some of the sensors is directly responsible for introducing an AC component in their Thevenin source without relying on the capacitance variation.