The Maxwell Commutated dc Bridge Reexamined

The Maxwell commutated dc bridge is analyzed, physically and mathematically. It is shown to have advantages as a signal processing circuit capable of reflecting the value of a capacitive transducer in proportional values of an active variable, current or voltage, and of providing indication or doing work. In particular, the circuit is shown to be applicable to capacitive unknowns having significant, finite series or parallel resistances; this contrasts with the contention of the classical references on the circuit. Also, the formerly widely used driven switch mechanism is replaced by a crystal-oscillator-driven solid-state switch, thus enhancing the reliability of the system. Experimental results for a representative circuit with both ideal and lossy capacitive unknowns conclude the paper.