The experiments now to be described have for their object the investigation of the magnetic induction in an iron conductor under the influence of a current passing through it. The method of experiment was briefly in this wise. An iron tube was magnetised circularly by a current passed from end to end along its entire length; and the induction so produced in the iron was measured in terms of the current induced in a coil of wire wound longitudinally round the walls of the tube.

In the experiments four tubes were used, all of the same length and nearly the same external diameter. The internal diameters of one pair were approximately double those of the other. The various dimensions are given accurately in the following table, the tubes being distinguished as A, B, a, b. Each diameter measurement is the mean of eight measurements taken across different diameters.