348 RICHARDSON— DYNAMICAL EFFECTS OF [April 22, 



removed. There will be no transportation of electrons and that is 

 what, on the electron theory, constitutes an electric current. The 

 electric field across the slab is, nevertheless, dift'erent from what 

 it would be if the material were not present. The difference be- 

 tween dift'erent insulating materials in this respect depends solely 

 on the comparative ease of displacement of the electrons they con- 

 tain. The specific inductive capacity of dielectrics, which, you will 

 remember, was discovered by Cavendish and Faraday, is, in fact, a 

 measure of the product of the number of electrons in unit volume 

 of the material b_\- the average displacement which they undergo in 

 unit field. 



The behavior of the second kind of material is quite dift'erent. 

 Even in the absence of the electric field, the so-called free electrons 

 are moving about in it in an irregular manner in all directions. The 

 effect of an external field is to superpose on the irregular motion a 

 definite drift, on the average, in the direction of the current. This 

 drift of the electrons involves traiisportafion of electricity and there- 

 fore implies the existence of an electric current. 



All the laws which regulate the transference of electricity across 

 conductors, such as, for example, Ohm's Law, which states that the 

 current is proportional to the applied electromotive force, and Joule's 

 Law, which states that the rate of production of heat by a current 

 is equal to the product of the resistance of the circuit by the square 

 of the current, follow at once from this simple hypothesis. It is not 

 necessary to suppose that all the electrons in the material are present 

 in the free condition ; some of them may be, and in all probability 

 the majority are, in a state of equilibrium similar to that which occurs 

 in insulators. All that is necessary is that some of the electrons 

 should be able to move without restraint. When the other conditions 

 are the same the magnitude of the current which a given material 

 will transport is proportional to the number of carriers available; 

 that is, to the number of free electrons per unit volume. 



It is in the explanation of the relation between the conductivity 

 of substances for electricit}- and for heat that the electron theory 

 has scored one of its most notable triumphs. Everybody knows that 

 the best conductors for electricity are also the best conductors for 

 heat. It is not so generally known how very close the relationship 



