Variation of Magneto-resistance Effect with Direction of the Magnetic Field.—(1) Theoretical discussion. The fact that in general the resistance of a conductor differs slightly according to whether the magnetic field is transverse or longitudinal, may be due to two factors. First, there may be a direct effect of the field on the motion of the electrons, as predicted by the electron theories of J. J. Thomson and others. But the author shows that if Townsend's method of developing the electron theory of conduction is adopted, the variation of resistance comes out zero. Second, the effect of the magnetic field may vary with the arrangement of crystals in the conductor, and therefore with the position of the conductor if the crystal structure is anisotropic. (2) Experiments with cost bismuth, pressed graphite and rolled cadmium are described in which a small bar or sheet of the material was placed in each of three mutually perpendicular positions in a field of 7,000 or 8,000 gausses, and resistance measurements were made for various positions of the rotatable Weiss magnet. For bismuth and cadmium the variations found are not symmetrical around the current direction and are evidently due chiefly to the crystal structure. For graphite the variations when the field was rotated in the plane of the sheet were within the experimental error, therefore both factors were inappreciable. These results are not conclusive but they suggest that the Townsend theory is correct. If so this magneto-resistance effect may be completely explained by assuming a change in the number and mean free period of the conducting electrons which depends not only on the magnetic field but on its direction with reference to the crystal axes.

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