Direct current comparators

A unique feature of the current comparator is that it is capable of being used with direct currents. Thus the high accuracy and stability of turns on a magnetic core, which are the basis of many high-quality alternating current instruments, is also made available to direct current measurements. Sensitivity to direct current ampere-turns or flux is achieved by modulating the magnetic core with superimposed alternating current ampere-turns, with the result that, when a direct current magnetising force is present, even harmonic components of the modulation are generated. Two identical magnetic cores are normally used so that by suitable interconnection of the windings on each core the odd harmonic components tend to cancel one another and only the even harmonics remain.