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The principle of magnification, as used in the special balance for studying magnetisation, and as described in Part I, Section (ii) of this paper, may be very conveniently applied for measuring small changes in length. By attaching one end of the rod in which the magnetostriction is to be examined to the diaphragm of the balance (4), fig. 1, Part I, and fixing the other end rigidly, a change in length of the rod when magnetised will displace the diaphragm and this displacement will be recorded on the photographic plate with a large magnification. A balance similar in dimensions to that used for susceptibility measurements has a magnification of about 105 times, and since we can easily distinguish deflections of 0·2 mm. on the photographic plate this will correspond to a possibility of measuring changes in length of 2 × 10-7 cm. in the rod examined. In actual practice some precautions have to be taken when using the balance as an extensometer of such high sensitivity. The main difficulty occurs in fixing the free end of the rod rigidly. Should it be absolutely rigid, even a very small thermal expansion in the rod caused by accidental temperature variation will produce a considerable deformation of the diaphragm, and unknown and uncontrollable strains will develop in the rod as well as in other parts of the apparatus; this will make the conditions of the experiment indeterminate. The difficulty is easily avoided by taking advantage of the short time of experiment. Instead of fixing the end of the rod rigidly, we attach it to a massive piston moving with great friction in oil; it is obvious that during 1/100 of a second, owing to the inertia of the piston and the large friction produced by the viscosity of the oil, the fixed end of the rod will be practically motionless, and the displacement of the diaphragm will be equal to the change of length of the rod. On the other hand, during the comparatively long interval between the experiments, the accidental changes in length due to thermal expansion will be compenated by the displacement of the piston, and the load on the diaphragm and the strain on the rod will be constant and the same in each individual experiment.

The study of the magnetic properties of matter in strong magnetic fields. Part IV.—The method of measuring magnetostriction in strong magnetic fields