Two new designs of `field mill´ type fieldmeters not requiring earthing of rotating chopper

Two novel designs of field-mill-type fieldmeters that have been devised and developed to avoid the problems which arise with the earthing of rotating choppers of traditional field mill instruments are described. The first design uses a second fieldmeter section to measure and compensate for the voltage on the rotor of the primary section; the second uses a constant-capacitance rotor arrangement with capacitive pick-up of the electric field modulated signal at the rotor. The two designs offer a measurement performance approaching conventional field mill instruments. The principles and features of the two new designs are described, and both avoid the need for earthing of the rotating chopper. These approaches enable long-term continuous and fast-response electric field measurements to be made with good zero stability and high sensitivity