Absolute Determination of the Ohm

Abstract

THE recent great advances in methods of calculating coefficients of mutual and self-induction, due to Prof. Jones and his colleagues in the work, has suggested that a method, which occurred to me some years ago, for the absolute determination of resistance might now be employed with, I think, some advantages over even the Lorentz method. It is as follows:—Take an earth inductor coil, fixed so that it can revolve continuously, thus giving a pure sine voltage, due to cutting the lines of the earth's magnetic field. Take the coils made by the Committee for the determination of the ampere, as described by Prof. Ayrton, and use the two outside coils as the primary and the inside coil as the secondary of a transformer. Pass the current from the earth inductor through one set of coils of a Kelvin balance, and thence through the primary of the transformer. Then through the (non-inductive or easily calculated inductive) resistance required to be measured, thence back to the earth inductor. Connect the secondary of the transformer to the other set of coils of the Kelvin balance, and speed up the earth inductor till the Kelvin balance arm becomes horizontal, and the pull on the two sets of coils is equal. Then take out the resistance we are measuring, and speed up the inductor till the Kelvin balance comes to zero again.