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180 1. subject has been brought into prominence recently by the use which Mr. Larmor has made of moving electrons in his dynamical theory of the aether. The matter was investigated in 1881 by Prof. J. J. Thomson, who showed that a point charge moving so slowly that the electric displacement it carries is not sensibly disturbed generates magnetic force like a current element according to Ampere's rule; and by Mr. Heaviside, who investigated the matter more generally in 1889, and showed that in steady rectilinear motion at any speed less than that of light, the lines of displacement continue to be radial but are concentrated