Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/343

 performed in the present system, the poles of the electromagnet are exchanged, while in the dielectric no change takes place.

We must now consider the bearing of Hall's effect on the question as to whether magnetism is a rotatory or a linear phenomenon. If magnetism be linear, electric currents must be rotatory, and if Hall's phenomenon be supposed to take place in a horizontal strip of metal, the magnetic force being directed vertically upwards, and the primary current flowing horizontally from north to south, the only geometrical entities involved are the vertical direction and a rotation in the east-and-west vertical plane; and these are indifferent with respect to a rotation in the north-and-south vertical plane, so that there is nothing in the physical circumstances of the system to determine in which direction the secondary current shall flow, The hypothesis that magnetism is linear appears therefore to be inconsistent with the existence of Hall's effect. There are, however, some considerations which may be urged on the other side. Hall's effect, like the magnetic rotation of light, takes place only in ponderable bodies, not in free aether; and its direction is sometimes in one sense, sometimes in the other, according to the nature of the substance. It may therefore be doubted whether these phenomena are not of a secondary character, and the argument based on them invalid. Moreover, as FitzGerald remarked, the magnetic lines of force associated with a system of currents are circuital and have no open ends, making it difficult to imagine how alteration of rotation inside them could be produced.

Of the various attempts to represent electric and magnetic phenomena by the motions and strains of a continuous medium, none of those hitherto considered has been found free from