In a former paper I described certain experiments on the relations of magnetism and twist in iron and nickel, the chief results of which it may be well to give briefly here. When an iron or nickel wire is under the influence of longitudinal and circular magnetisations, it twists in a direction which is definitely related to the direction of the magnetising forces. This effect in iron was discovered by Wiedemann, and for convenience I shall call it the Wiedemann Effect. It was pointed out by Clerk Maxwell that the Wiedemann effect might be explained as a consequence of the earlier discovery made by Joule, that iron lengthens in the direction of magnetisation, and contracts at right angles thereto. Led by a consideration of Barrett's discovery of the shortening of nickel wire in the direction of magnetisation, I determined to test nickel in the same way in which Wiedemann had tested iron. It was quite obvious that, if Maxwell's explanation of the Wiedemann effect were the true one, nickel wire should, ceteris paribus, twist in a sense opposite to that in which iron twists. The experiment when made completely fulfilled the expectation. Thus, when an iron wire, with one end fixed, is traversed by an electric current in the direction in which it is at the same time longitudinally magnetised, the wire is twisted so that the free end rotates right-handedly with reference to the traversing current, or the longitudinal magnetisation. In nickel, on the contrary, the corresponding rotation is left-handed. This was the chief conclusion arrived at in my earlier paper; and a little consideration will show how very readily the Wiedemann effect, whether in iron or in nickel, is explained in terms of the simpler strains studied by Joule and Barrett.