﻿Mechanism of Electrical Conduction. 59 



the conductive particles are perfectly conductive, no electro- 

 magnetic energy can penetrate within them. Thus the energy 

 expended by the source of E.M.F. which maintains a steady 

 current through a conductor is converted partly into additional 

 energy of the molecules, and partly into electromagnetic dis- 

 turbances of the intervening ether : that is, the dissipated 

 energy takes the form of heat, as we know from experiment. 



7. Ohm's Law. 



In the case of a metal wire (especially one at a bright red 

 heat), Ohm's Law has been verified with great exactitude, 

 the results of the experiments designed by Maxwell and carried 

 out by Chrystal being summed up by the latter in the following 

 words *: — " If we have a conductor [of iron, platinum, or 

 German silver] whose section is a square centimetre, and 

 whose resistance for infinitely small currents is an ohm, its 

 resistance (provided the temperature is kept the same) is not 

 diminished by so much as the 1/10 12 part when a current of a 

 farad per second passes through it." 



Now when a current is conveyed through a substance by 

 intermittent contacts amongst a number of perfectly con- 

 ductive particles, the effective conductivity depends firstly on 

 the properties of the intermolecular medium, and secondly on 

 the size, form, distribution, and movements of the particles 

 themselves. In order that the resistance of the conductor 

 may be sensibly constant — in order, that is, that the current 

 transmitted may be sensibly proportional to the impressed 

 E.M.F. — two conditions must evidently be satisfied: — 



(i.) For such values of impressed electromotive intensity 

 as exist in the intermolecular spaces (say about *003 volt 

 per cm.) the relation between electromotive intensity and 

 electric displacement must be sensibly linear. 



(ii.) The forces which the particles of the substance ex- 

 perience owing to the impressed E.M.F. must be very small 

 in comparison with the ordinary intermolecular forces, so that 

 during the time of a single molecular excursion the motion 

 of no particle is appreciably influenced by the presence of the 

 E.M.F. If we suppose that in the conducting substance we 

 can maintain a steady distribution of temperature which is 

 independent of the current flowing through, this second con- 

 dition implies that the particles of the substance under the 

 steady distribution may be regarded as a system of perfect 

 conductors, whose coordinates are explicitly given functions 

 of the time, and are sensibly unalterable by an E.M.F. 



* B. A. Report, 1876, p. 61 of Reports. 



