Similar phenomena could be observed with a closed circular tube capable of rotating about its axis, and there must be a corresponding electromagnetic phenomenon if a metallic wire contains something like movable electricity, as we are now in a position to assert, because we have good reasons for believing that an electric current consists in a motion of negative electrons. Though the experiment is much more delicate with electricity than with water, Tolman and Stewart have performed it with very satisfactory results. Using a coil with a great number of windings, rapidly rotating about its axis, they were able to observe with a sensitive galvanometer the transient electric current that was produced on stopping the motion by means of a brake. The electrons continued to move over some distance just as the water did in the experiment with the circular tube. The direction of the current showed that the movable particles really have negative charges and the observed deflections agreed with what can be inferred from the ratio between the charge and the mass of the electrons, a ratio that was found for the first time by Zeeman and Sir J. J. Thomson somewhat more than twenty-five years ago and has been repeatedly determined in later years. No less remarkable than Tolman and Stewart's experiments are those made by Mr and Mrs Barnett. They found that a cylindrical rod of iron, rotating about its geometrical axis, becomes thereby magnetized in the direction of its length. Like the Einstein effect, to which it forms a counterpart, this new phenomenon can be predicated on the assumption that magnetization consists in a motion of electrons in the molecules of the metal. This allows us to assign the direction of the effects, but for the sake of truth I must add that both the rotation produced by the magnetization of rods and the magnetization caused by a rotation are, for some reason which we do not yet understand, only about half what the theory of electrons had led us to expect. In Maxwell's time the electron was unknown and the mechanism of conduction was even more mysterious than it is now. It was precisely for this reason that he attached so much importance to the phenomena to which I have drawn your attention. He says in this connexion: It appears to me that while we derive great advantage from the recognition of the many analogies between the electric current and a current of a material fluid, we must carefully avoid making any assumption not warranted by experimental evidence, and that there is, as yet, no experimental evidence to shew whether the electric current is really a current of a material substance, or a double current, or whether its velocity is great or small as measured in feet per second.