across a magnetic field will therefore be accompanied by the same reactions as if a procession of electrons were suddenly started in it. This, however, would involve at the moment of starting a backward push on surrounding electrons, just as when a boat is set in motion by oars the boat is pushed forward and the water is pushed back. Hence there is an induced current at the moment when the field begins in the conductor. Similarly the reaction at stopping the procession would drag the surrounding electrons with it. Accordingly the induced currents when the field ceases is in the opposite direction to that when it begins.
The electronic theory has in the hands of other theorists such as Professors P. Drude and E. Riecke been shown to be capable of rendering an account of most thermomagnetic effects on metals, contact electricity, the so-called Thomson effects in thermoelectricity, and also the Hall effect in metals when placed in a magnetic field.
Electrons and Æther.
The ultimate nature of an electron and its relation to the æther has engaged the attention of many physicists, but we may refer here more particularly to the views of Dr. J. Larmor whose investigations in this difficult subject are described in his book on 'Æther and Matter' and also in a series of important papers on the transactions of the Royal Society of London, entitled 'A Dynamical Theory of the Electric and Luminiferous Medium' (Phil. Trans. Roy. Soc, 1893, 1895, 1898). Larmor starts with the assumption of an æther which is a frictionless fluid, but possesses the property of inertia; in other words he assumes that its various parts can have motion with respect to each other and that this motion involves the association of energy with the medium. He regards the electron as a strain center in the æther, that is as a locality from which aether strain radiates. Electrons can therefore be either positive or negative according to the direction of the strain and to every positive electron there is a corresponding negative one. Atoms according to him are collocations of electrons in stable orbital motion like star clusters or systems.
An electron in motion is in fact a shifting center of æther strain and it can be displaced through a stationary æther just as a kink or knot in a rope can be changed from place to place on the rope.
An electron in vibration creates an æther wave but it radiates only when its velocity is being accelerated and not when it is uniform.
The type of æther which Larmor assumes as the basis of his reasoning is one which has a rotational elasticity, that is to say, the various portions of it do not resist being sheared or slid over each other, but they resist being given a rotation round any axis. Starting from these postulates and guided by the general and fundamental principle of Least
