rotary voltmeter[′rōd·ə·rē ′vōlt‚mēd·ər]
an instrument for measuring high DC and steady-state AC voltages.
The rotary voltmeter—an instrument in an electrostatic system—is based on the reciprocal action of electrically charged metallic bodies. There are two types of rotary voltmeters, rotary and generating. Rotor voltmeters are used to measure DC voltage, as well as the amplitudes and instantaneous values of steady-state AC voltages. The voltage to be measured is supplied to two stationary electrodes; a rotor, which is divided into half-cylinders that are insulated from the shaft of the rotor and from each other, rotates evenly in the electrical field of the electrodes. The rectified current is measured by a galvanometer through a commutator and brushes. Generating rotary voltmeters are intended for the measurement of DC voltage. They are of various designs; they have no commutator, and the current is supplied directly through the rectifier to the galvanometer.