339-342] 



Physical Principles 



297 



In general, however, the resistance of a conductor varies with the temperature, and 

 for some substances, of which selenium is a notable example, it varies with the amount 

 of light falling on the conductor. In gases the resistance varies so much with all the 

 physical conditions, including the amount of current passing, that Ohm's Law can 

 hardly be said to have any meaning. But in gases, a current is conveyed by a process 

 different in many respects from that which goes on in a conductor of the usual kind. 



The Voltaic Cell. 



342. The simplest arrangement by which a steady flow of electricity can 

 be produced is that known as a Voltaic Cell. This is represented diagram - 

 matically in Fig. 95. A voltaic cell consists essentially of two conductors 



FIG. 95. 



A, B of different materials, placed in a liquid which acts chemically on at 

 least one of them. On establishing electrical contact between the two ends 

 of the conductors which are out of the liquid, it is found that a continuous 

 current flows round the circuit which is formed by the two conductors and 

 the liquid, the energy which is required to maintain the current being 

 derived from chemical action in the cell. 



To explain the action of the cell, it will be necessary to touch on a subject 

 of which a full account would be out of place in the present book. As an 

 experimental fact it is found that two conductors of dissimilar material, when 

 placed in contact have different potentials when there is no flow of elec- 

 tricity from one to the other*, although of course the potential over the 

 whole of either conductor must be constant. In the light of this experi- 

 mental fact, let us consider the conditions prevailing in the voltaic cell before 

 the two ends a, b of the conductors are joined. 



So long as the two conductors A, B and the liquid G do not form a closed 

 circuit, there can be no flow of electricity. Thus there is electric equilibrium, 



* For a long time there has been a divergence of opinion as to whether this difference of 

 potential is not due to the chemical change at the surfaces of the conductors, and therefore 

 dependent on the presence of a layer of air or other third substance between the conductors. It 

 seems now to be almost certain that this is the case, but the question is not one of vital 

 importance as regards the mathematical theory of electric currents. 



