Changes of Length in Bars and Wires of Magnetic Material Due to Magnetisation

Abstract

THE appearance of a paper, by Dr. L. T. More, on the changes in length produced in iron wires by magnetisation (Phil. Mag., October 1895, P. 345, and Physical Review, vol. iii. p. 210), has drawn my attention to a curious divergence of opinion on a fundamental point in magnetism. Dr. More has attempted to analyse the change of length accompanying magnetisation into a “direct action” which “may possibly be due to the orienting of the magnetised particles,” and to “indirect actions.” He adds (p. 224): “These indirect actions are the mechanical stresses created in the rod by the magnetism. The first of these is the tractive force of the magnet and is measured by B²/8π. That this force exists, tending always to contract the rod (italics mine), is seen from the fact that if the magnet is cut in two, the ends are held together.… This effect for high intensities of magnetisation is a large one, and becomes one of the most important factors in the observed changes in length.” The stress referred to by Dr. More is that usually associated with the name of Maxwell (“Electricity and Magnetism,” vol. ii. Arts. 641 et seq. Cf. Ewing, “Magnetic Induction in Iron and other Metals,” § 147).