Magnetic induction and its reversal in spherical Iron shells

Abstract

In a previous communication to the Society, an account was given of an investigation of the magnetic shielding of a large space, by the use of a series of spherical iron shells. The basis of this investigation, which is still in progress, is strictly quantitative, and the work has been retarded, to a great extent, by the fact that several processes adopted commonly by various investigators of magnetic phenomena have never been examined accurately on the theoretical side, and in several cases even on the experimental side, to an order of precision which is sufficient for delicate measurements. One of the main objects ultimately in view, in the work described already, is the production, throughout the volume of a large iron specimen, of a magnetic field so small, in comparison with that of the earth, for example, as to render possible some accurate conclusions regarding the behaviour of iron under indefinitely small magnetic forces. This behaviour is at present a matter of doubt, and it remains to be shown that the indications of its real nature, obtained in earlier papers by one of us, continue to point in the same direction when the force is decreased below any values which have been employed hitherto. In the experiments described in the former paper, a field so low as 0·0033 C. G. S. unit was obtained in a spherical space of radius 32·5 cm. by the use of four concentric shells. It was accompanied, however, by a leakage field of 0·006 C. G. S. unit, and the shells still showed some traces of permanent polarisation after continued treatment with many reversals of a slowly decreasing current. This method of demagnetisation is used extensively, and in its application coils have sometimes been wound somewhat loosely on the iron to be demagnetised, and the constant field given by Maxwell’s formula for a spherical internal space surrounded by a very closely wound coil assumed to exist. But if a thin shell is wound in this manner, although the more internal parts may be subject to this field, the results must be widely different in the outer material of the shell. Further experimental work on the improvement of magnetic shielding beyond the order obtained already— in fact by such methods as the employment of a very small magnetising current to remove, in the internal space, the small part of the earth’s field which may remain after the operation of the shielding effect of a set of shells —requires a precise knowledge of the value of the magnetic induction in a shell, or one member of a set of shells, wound by a spherical helical coil. For the condition of indefinitely close packing is found to be essential to the success of Maxwell’s formula, which—but only from this particular point of view—does not appear, either from an a priori theoretical consideration, or from the experimental results described in the present paper, to have the large range of validity which has been assumed for it hitherto by practical workers on magnetic phenomena.