an automatic measuring device whose principle of operation is the balanced method of measurement—that is, a method in which two values, usually voltages or currents, are compared. Automatic compensators have either a servomotor or a compensation amplifier.
In automatic compensators with a servomotor a sensor converts the value x being measured into an electromotive force (emf) Ex, which is compared to another emf in the balancing circuit. The error signal ΔE is amplified and causes rotation of the servomotor. The balancing member is simultaneously displaced, changing the resistance of the compensating circuit (the balancing network) in such a way as to decrease ΔE; when ΔE = 0, the servomotor stops. Readings are made from an instrument pointer or by an automatic recorder rigidly connected to the balancing member. Such devices as automatic potentiometers operate according to this principle. Automatic compensators with a servomotor may be classified as closed astatic systems, since they contain one integrator (the motor); such systems are a type of servomechanism. Most devices that operate according to this principle are capable of recording— and sometimes also regulating—the value being measured. Such self-balancing instruments may be used to measure any value (emf, resistance, inductance, capacitance, frequency, power, temperature, mechanical displacement, pressure, level, light flux, and so on) that can be converted into an electric signal. The advantages of such instruments are high sensitivity (the sensitivity threshold of some automatic compensators is less than 1 microvolt), high measuring accuracy (0.2–0.5 percent of total scale range), the ability to perform measurements without consumption of power from the circuit being measured and to perform measurements at remote points, and very rapid response (displacement speed of up to 1 m/sec along the scale).
In automatic compensators with a compensation amplifier the emf Ex being measured is compared with a compensating voltage Ex The error signal ΔE = Ex - Ec is fed to the input of the amplifier, at whose output current I appears, generating a compensating voltage IR = Ecy which is approximately equal to Ex. The measurement results are read on a galvanometer. Automatic compensators with a compensation amplifier are closed and generally static systems, since there is no integrator. Such systems are characterized by a static or compensation error (Ex — Ec). Such an automatic compensator can also be regarded as a measuring amplifier with feedback that converts a small emf Ex fed to its input into a proportionally variable current I. The measurement accuracy of this type of automatic compensator is limited by the precision grade of the galvanometer. Automatic compensators with compensation amplifiers are mainly laboratory instruments for measurements of small constant emf s and currents and for calibration of electrical measuring instruments; they are sometimes used as complex AC compensators.