The absolute measurement of electrical resistance by a method using the average electromotive force of a commutating generator
H Redmayne Nettleton, E G Balls · Proceedings of the Physical Society · 1942
An electrical resistance of approximately 1 ohm is measured absolutely with rapidity and precision in terms of a mutual inductance and a frequency, by balancing the e.m.f. across it when conveying direct electric current, against the average e.m.f. of a commutating generator, the field coils of which are traversed by the same current. By a new method, a remarkable flat of mutual inductance is obtained between the rotor and the field coils over a range of 20° of arc in the neighbourhood of the commutations. Further, over an adjustable portion of this range, covering the actual breaks at commutations, the rotor together with additional adjustable resistance is short-circuited, thus enabling the e.m.f. across the resistor to suffer no interruption in its supply to the detector and allowing it to supply the extra quantity of electricity absorbed by the rotor, in virtue of its self-inductance, on establishing the current through it at each commutation. In this way the two main errors associated with break and self-inductance are set against one another, and compensation is readily effected by the adjustment of a resistance in a preliminary experimental test carried out under conditions in which these two effects preponderate. The rotor has a constant speed of 12.5 revolutions per second, and the maximum mutual inductance of some 20,000 μH. permits of rapid and fine adjustment over a range of 400 μH. The balance is sensitive to 0.1 μH. and an accuracy of a few parts in a hundred thousand has been obtained.