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VII. On the Magnetisation of Iron and other Magnetic Metals in very Strong Fields. 
By J. A. Ewing, B.Sc., F.R.S., Professor of Engineering in University College, 
Dundee, emd William Low. 
Received October 29,—Read November 22, 1888. 
§ 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.t 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 mag¬ 
netisation 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 con¬ 
tinuous 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 w^as so small that the external field 
* “ On the Magnetisation of Iron in Strong Fields,” ‘ Roy. Soc. Proc.,’ vol. 42, p. 200. 
t Preliminary notices of some of the later results were communicated to Section A of the British 
Association at Manchester (‘ Report of the British Association for 1887,’ pp. 586 and 587). 
6.4.89. 
