210 THE REALITIES OF MODERN SCIENCE 



potential energy of the system is not, however, as 

 in a gravitational system, inherent in the separa- 

 tion of the two bodies. It is a potential energy of 

 ponderable and visible masses by appearance only. 

 In final analysis, it is a kinetic energy of the electrons 

 within the conductors, for unless there are electron 

 streams there is no potential energy. When motion is 

 allowed, external work is done at the expense of the elec- 

 trons. 



The current is therefore reduced. This is not, 

 however, the only reduction which occurs. Due to 

 the relative motion of the two conductors, currents are 

 induced in them in such directions as to oppose the 

 currents which occasion this motion. The average 

 velocity of the free electrons which are taking part in 

 the conduction is therefore reduced. (An induced 

 current occurs whether the motion inducing it is caused 

 by an expenditure of mechanical energy or is the result 

 of the mutual attraction of the two parallel currents.) 



If the electron streams are not allowed to decrease, 

 but are maintained at the expense of batteries or other 

 sources of energy connected in the conducting circuits, 

 then such sources must supply an amount of energy 

 equal to the external work accompanying the motion, 

 and also such energy as is required to restore the elec- 

 trons to their former velocities. (We are neglecting 

 any stopping of the electrons due to collisions in the 

 conductors whereby energy is dissipated in heat.) 



Let us state this phenomenon in a slightly different 

 manner. Two conductors carrying parallel currents 

 possess a potential energy which depends upon the 

 currents and is less if the separation between the con- 



