Current-Difference Circuit and its Application to Electrolytic Conductivity Measurement

Variations of a current-difference circuit for making impedance measurements were investigated theoretically and several modifications were tested. The circuit is based on the use of a dc meter and four diode rectifiers. It can compare two impedances, while maintaining alternating current flow through both impedances. The latter characteristic makes the circuit applicable to the measurement of resistance of electrolytic conductivity cells where asymmetric polarization is to be avoided. The comparison is not inherently dependent on a ratio established by a pair of resistors or by a pair of transformer windings. The circuit can compare impedances of similar phase angle, for example, two resistors, two inductors, or two capacitors. It can also compare impedances or admittances of different phase angle. This latter characteristic is of value, for example, in the measurement of the resistance component of electrolytic conductivity cells where there may be a small series reactance component as the result of symmetrical polarization. The precision of this simple circuit appears satisfactory for many practical applications of electrolytic conductivity measurement. The utility of the circuit is based to a large extent on the good characteristics of diodes now available.