The absolute measurement of electrical resistance by new rotary coil methods

Abstract

PART I contains a description of the writer's further work on the absolute measurement of electrical resistance by the new rotating coil method of Nettleton and Balls. The preliminary investigation was published in 1934. In the fundamental equation, K, a constant, depends upon where M is the mutual inductance between rotor and stator at angle of rotation from the conjugate position where the brush is at the commutator sector's edge. K is shown to be most accurately determined at and with the field coils in the Helmholtz position. To satisfy these conditions and to render constant at the sector edges, it is necessary to construct a differential rotor consisting of two coplanar coils in series and opposition. The theory of this rotor necessitated the calculation of mutual inductances and Tables of Functions, given in Appendix II. With such a rotor, resistances of the order of one ohm are measured absolutely. above, is the angular velocity of revolution and is measured by the method described in Appendix I. Part II is a preliminary account of the writer's investigation, suggested by Dr Nettleton, on the absolute measurement of resistance by the method, proposed by Rosa in 1909, using the e.m.f. of a commutating generator. No data by any form of this method have ever been published. The basic equation of this method, has the great advantage that a frequency and the maximum mutual inductance only are required. A remarkable flat of maximum mutual inductance between rotor and stator was obtained, permitting commutation without loss of generated e.m.f. Rosa's difficulties, due to self-inductance and to break at commutation in the steady e.m.f. drawn from the resistance to be measured, are overcome by a short-circuiting device covering each commutation.