ELECTRONIC THEORY OF ELECTRICITY. 19 



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 cur- 

 rents 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 render- 

 ing 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 JEther. 



The ultimate nature of an electron and its relation to the aether 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 'iEther 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 aether 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 aether, 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 aether strain and 

 it can be displaced through a stationary aether 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 aether wave but it radiates only 

 when its velocity is being accelerated and not when it is uniform. 



The type of aether which Larmor assumes as the basis of his reason- 

 ing 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 



