Electrostatic Instrument

a measuring instrument whose principle of operation is based on the mechanical interaction of electrodes that carry electric charges, most often, opposite electric charges.

Figure 1. Electrostatic instrument: (1) movable electrode, (2) fixed electrode, (3) shaft, (4) spring, (5) pointer, (6) scale

The quantity measured in an electrostatic instrument is converted to either AC or DC voltage, depending on the type of electrostatic mechanism that is used. The voltage being measured in an electrostatic instrument such as that illustrated in Figure 1 is applied to a movable electrode, which is fastened to a shaft to which a pointer is also attached, and to a fixed electrode, which is insulated from the first. As a result of the interaction of the charges that appear on the electrodes, a torque develops on the shaft in proportion to the square of the applied voltage. A spring acts on the shaft to produce an opposing torque that is proportional to the angle of rotation of the shaft. In the interaction of the torques, the pointer rotates through an angle that is proportional to the square of the applied voltage. The instrument’s scale, which is calibrated in units of the quantities being measured, is nonuniform and often has a light-beam indicator.

Electrostatic instruments are usually used to measure AC or DC voltages that may include high-frequency voltages. They draw little power, and their readings are independent of frequency. Because electrostatic instruments are sensitive to external electrostatic fields, they are equipped with internal shielding. The class of precision of electrostatic instruments is the highest possible, that is, 0.005.