VII. On the magnetisation of iron and other magnetic metals in very strong fields

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Abstract
§ 1. Early in 1887 we communicated to the Royal Society a short account of experiments made to examine the magnetic behaviour of iron when subjected to strong magnetic force by what we called the “isthmus” method of magnetisation. Since then the experiments have been continued and extended by applying stronger magnetic forces, and by testing samples of nickel, cobalt, and various steels, as well as wrought iron and cast iron. It may be well to preface an account of the more recent experiments by a short summary of the results stated in our earlier paper. § 2. The method of experiment consisted in turning the piece of metal whose magnetisation was to be examined to the form of a bobbin with a narrow neck or isthmus, and placing that between the pole-pieces of a powerful electromagnet. The sample was furnished with a spreading cone at each end, to facilitate the convergence of the lines of magnetic induction upon the central neck. The magnetisation was measured by means of an induction coil of fine wire wound in a single layer, or, in some cases, in two layers, upon the metal of the neck. Outside of this coil, and at a small definite distance from it, a second induction coil was wound in order to measure the magnetic field in the space between the two coils. This served a double purpose: it enabled a proper deduction to be made from the values of the induction measured within the inner coil, to allow for the space between the surface of the iron neck and the centre of the thickness of the coil; and it gave values of the magnetic force in the space immediately surrounding the iron, from which an inference might be drawn as to the value of the force within the neck itself. As there was no free magnetism on the circumference of the neck, in the medial plane, the force within the metal was continuous there with the force outside, and it will be shown later that when a suitable slope was given to the conical ends of the bobbin the variation of force in the medial plane in directions at right angles to the axis was so small that the external field must have formed (in such cases) a very approximately accurate measure of the force acting on the metal. In other cases, when the cones were more blunt, the force in the external field was somewhat greater than the mean force within the metal.

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