192 THE REALITIES OF MODERN SCIENCE 



Before considering them let us summarize the factors 

 that enter into the conduction of electricity between 

 two plates or electrodes which are maintained by some 

 means or other at a constant difference of potential. 

 The current at any instant depends upon the number 

 of carriers which are available and upon the velocity 

 with which they are "falling." The velocity will 

 depend upon the potential difference between the two 

 electrodes, but the number of available carriers will 

 depend upon several other factors. Thus it will de- 

 pend upon whether or not the conducting path is 

 ionized, and this in turn depends upon what the medium 

 is and upon whether or not the potential gradient has 

 been sufficient. If the path is ionized the number of 

 available carriers will depend upon its previous history, 

 for it was the carriers which were present in the pre- 

 ceding instant which were active in forming those now 

 available. The number of carriers will also depend 

 upon the character and electronic condition of the neg- 

 ative electrode and upon the energy which the electrode 

 or the conducting path may be receiving in the form, 

 for example, of ultra-violet radiations, or X-rays. 



In the case of metals, however, the supply of carriers 

 is apparently equal to any demand which may be made 

 and, provided the temperature of a metal conductor, 

 and hence the mean free path, is maintained constant, 

 the current depends only upon the potential difference. 

 An increase in the current is accomplished by increas- 

 ing the average velocity with which the electrons 

 travel toward the positive plate and hence the num- 

 ber per second which crosses any area of the conducting 

 path. Let us see how the potential difference must 



