A three-terminal capacitor having high and low voltage electrodes coaxially aligned in a housing which serves as a ground plane includes a non-gaseous dielectric material filling the cavity of the housing. In the preferred embodiments, the dielectric is either a suitable oil or plastic. A source of voltage, that can be used to power for example an external electrical measuring device, can be generated by arranging a tertiary electrode proximate to the high voltage electrode and suitably selecting the ratio of surface areas of the tertiary and high voltage electrodes to produce sufficient current to power the device. Various embodiments of the capacitor also include and internal current transformer which provides a current signal indicative of series current flow through the capacitor. This current signal and the signal appearing on the low voltage electrode can be used to readily provide an indication of the amount of power supplied by a power transmission line to a load situated downstream of the capacitor.