Two precision condenser bridges
Albert Campbell · Proceedings of the Physical Society · 1931
In Heydweiller's modification of the Carey Foster bridge as ordinarily used (with variable mutual inductance) the capacitance of the condenser can be read directly, but the power-factor has to be deduced by calculation. To facilitate direct reading of both capacitance and power-factor the author has developed two bridge systems, A and B , based on that of Carey Foster. In both of these a fixed mutual inductance M is used and there is no added resistance in the condenser arm. The capacitance C eq, falling dots M/PR , where P and R are the resistances of the other two arms. To give direct reading for C, P can be varied in simple steps (e.g. 10, 100, 1000, 10,000), giving range multipliers, while R consists of a conductance box reading in millimhos. In system A a fourth arm Q of variable low resistance is added to the bridge. The power-factor is given by Q/M ω and can be read directly (at given pulsatance ω) if a slide wire is used for Q . In system B the power loss in C is balanced by the addition of impurity to the mutual inductance M by means of a closed-loop circuit variably coupled to both the primary and secondary coils of M . When the resistance of this loop circuit is set proportional to the frequency, the scale of the double inductor which varies the couplings can be graduated to read the power-factor directly. When a simple amplifier is used in the detector circuit, a capacitance range of 100μμF up to 10μF can be obtained and a power-factor range from 0.0001 to 0.01 with high accuracy of reading.