III. 



THE DYNAMICAL EFFECTS OF AGGREGATES OF 

 ELECTRONS. 



By OWEN W. RICHARDSON. 

 (Read April 22, 1911.) 



I. Electrons and Matter. 



The enormous difference in tlie behavior of dift'erent materials 

 towards electric force is a matter with which everyone is familiar ; 

 and it is one of the triumphs of the electron theory that it has given 

 us a verv satisfactory picture of the dift'erence between insulators 

 and conductors of electricity. We are to regard all matter as made 

 up primarily out of electrons. They are the stones with which the 

 material structure is built up, the electrodynamic forces are the 

 cement which holds the stones together. There are, however, two 

 different ways in which the electrons may exist in a given portion 

 of matter. They may be located in position of stable equilibrium, 

 in which case a very small force will displace them to a small extent 

 but a perfectly enormous force would be required to dislodge them 

 thoroughly and give rise to instability ; or they may be so loosely 

 held that they are able to move about in the interstices of the material, 

 very much after the fashion in which we believe the molecules move 

 about in a gas. In the former case, when the electrons are practically 

 fixed, we say the substance is an insulator ; in the latter case, where 

 they are wandering about, the substance is a conductor. 



A moment's reflection will show that this dift'erence is sufficient 

 to explain the difference between insulators and conductors. Con- 

 sider what happens when a slab of the first kind is placed in an 

 electric field. There will be a displacement of the electrons, it is 

 true, but the displacement will be small and they will all return to 

 their original equilibrium positions as soon as the external field is 



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