Substitution Method for the Determination of Resistance of Inductors and Capacitors at Radio Frequencies

The use of a series resonant circuit for the measurement of the resistance of coils and capacitors at high frequencies is discussed. The circuit is tuned with the unknown in circuit and with it removed, introducing sufficient resistance to keep the current constant. From the condition of constant current, and the circuit constants, the constants of the unknown are derived. The derivation of the expression for the resistance of a capacitor involves certain assumptions which are shown to be justified for frequencies at which coils and capacitors are now generally used. Sources of error are pointed out and discussed. In the measurement of resistances of the order of two ohms and larger, the precision of the method is of the order of one per cent. A change of circuit resistance of approximately 0.02 ohm can be measured.