176 Dr. J. A. Fleming [May 30, 



the equivalent of the exhaustion of the chemical potential energy of 

 the cell. Thus the electronic theory outlines for us in a simple 

 manner the meaning of voltaic action. Even if we do not admit 

 the existence of a metallic junction volta contact force, the theory 

 of the cell may be based on the view that the movement of the 

 saline ions in the electrolyte is determined by the law that that 

 motion takes place which results in the greatest exhaustion of 

 potential energy. Hence the chlorine ions move to the zinc and not 

 to the copper. 



In the same manner the electronic theory supplies a clue to the 

 explanation of the production of an electric current when a conductor 

 is moved across a magnetic field. Every electron in motion creates a 

 magnetic force. Hence a uniform magnetic field maybe considered as 

 if due to a moving sheet of electrons. The ' cutting ' of a conductor 

 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 pro- 

 cession would drag the surrounding electron with it. Accordingly 

 the induced current 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. Biecke been known 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 ^iher. 



The ultimate nature of an electron and its relation to the BBther 

 has engaged the attention of many physicists, but we may refer here 

 more particularly to the views of Dr. J. Larnior whose investigations 

 in this difficult subject are described in his book on '^ther and 

 Matter ' and also in a series of important papers in the ' Transactions ' 

 of the Koyal Society of London, entitled ' A Dynamical Theory of 

 the Electric and Luminiferous Medium.' * Larmor starts with the 

 assumption of an eether 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 centre in the aether, that is as a locality from 

 which rether strain radiates. Electrons can therefore be either posi- 



* Phil. Trans. Roy. Soc., 1893, 1895, 1898. 



