Magnetic Shielding in Hollow Iron Cylinders

This investigation deals with the shielding which exists within hollow iron cylinders when placed in a uniform transverse magnetic field. This transverse field divided by the internal magnetic field is defined as the shielding ratio. For the thin iron cylinders experimented with, it appears from various mathematical contributions to the subject that the shielding ratio minus unity may for present purposes be taken to be proportional to certain geometrical data and to the permeability (the permeability being large). By means of a rotating inductor within the shield, connected with a ballistic galvanometer, experimental determinations of the shielding ratio are made under various conditions of magnetisation; and an endeavour is also made to show how far and under what conditions these results approximate to theoretical formulas which assume the permeability to be uniform all round the shield, and the absence of retentivity and coercive force in the iron. Two iron shields were experimented with, the hysteretic constants (η) being ·0015 and ·0028 respectively.

II. On the refraction-equivalents of the elements

In our paper “On the Refraction, Dispersion, and Sensitiveness of Liquids,” Mr. Dale and I pointed out a property of bodies which we termed their “specific refractive energy.” It is the refractive index minus unity, divided by the density, or in symbolical language μ -1/ d . We found that this is a constant unaffected by temperature, and that the specific refractive energy of a mixture is the mean of the specific refractive energies of its constituents. At the same time, however, we admitted that in both cases our numbers were not in perfect accordance with theory, there being some unknown cause which affected them to a slight extent. These conclusions, both in regard to the general law and its qualification, have been since confirmed by continental physicists, and especially by the late rigorous experiments of Wullner. In the same paper we ventured also on the generalization that “every liquid has a specific refractive energy composed of the specific refractive energies of its component elements, modified by the manner of combination.” Later research has confirmed this also, extending it to conditions of matter other than liquid, and showing more clearly when such modifications occur, and what is their nature. Professor Landolt, of Bonn, has greatly advanced our knowledge of the subject, and has simplified the calculations by adopting what he terms the refraction-equivalent, that is, the specific refractive energy multiplied by the atomic weight, or P μ -1/ d . Recent investigations in fact tend to the general conclusion that the refraction-equivalent, not only of mixtures, but of every com­pound body, is the sum of the refraction-equivalents of the elements that compose it.

III. Researches on refraction-equivalents

Since the paper of the Rev. T. Pelham Dale and myself “On the Refraction, Dispersion, and Sensitiveness of Liquids our researches have been continued from time to time, and a good deal of attention has been paid to the subject in Germany. The permanence of the specific refractive energy of a body, notwithstanding change of temperature, aggregate condition, solution, or even chemical combination, has been confirmed, and upon this has been built the doctrine of Refraction-equivalents. Our specific refractive energy is the refractive index of any substance minus unity, divided by the density; in symbolic language µ –1/ d . Professor Landolt’s “Refraction-equivalent” is the same multiplied by the chemical equivalent, or P µ –1/ d .