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A circuit is described by which electromotive forces can be compared accurately without making any direct electrical connection between them. A double-pole, double-throw chopper, having a condenser connected between the vibrating contacts, is used with a galvanometer or electronic amplifier to indicate any inequality between the e.m.f.'s without allowing any net flow of current between the circuits. Polystyrene condensers and mechanically driven choppers are used to minimize leakage and pickup problems.Potentiometric measurements may be made through this "isolating potential comparator" to ± 0.1 μv. The error due to potential difference between the circuits is about 10−6 of the difference for 100 ohm circuits. An analysis of the method is given and optimum condenser and galvanometer characteristics determined. Magnitude and elimination of errors are discussed.The comparator is suitable for measurement or control of e.m.f. differences between thermocouples which are electrically connected through the tips. It also has many applications in resistance measurement and resistance thermometry.
T. M. Dauphinee (Fri,) studied this question.