A Method for Determining the Residual Inductance and Resistance of a Variable Air Condenser at Radio Frequencies

When using a variable air condenser as a reactance standard at radio frequencies it is important to know the variation in effective capacitance and power factor with frequency. The procedure outlined in this paper furnishes a method for the measurement of the residual impedances causing such variations. A qualitative discussion of the sources of both residual inductance and resistance is presented. The residual inductance is largely localized in the leads and stack supports. The residual resistance may be considered as made up of a conductance caused by dielectric loss and a resistance caused by ohmic and eddy-current loss. It is pointed out that all these residual parameters tend to remain constant at a single frequency, irrespective of capacitance setting. The procedure for determination of the residual parameters depends upon the measurement of a fixed condenser by a substitution method, using the variable condenser to be studied as standard. At a single frequency the residual inductance of the standard condenser causes a deviation in effective capacitance from the static value which depends upon the setting. The residual resistance caused by ohmic and eddy-current loss causes a variation in effective conductance which also depends upon the setting. If the capacitance and conductance components of the fixed condenser be measured over different ranges of the scale of the standard condenser there will therefore be an apparent variation in their values. Interpretation of these trends yields the magnitudes of the residuals causing them.