216 THE REALITIES OF MODERN SCIENCE 



ing upon the relative directions of the two currents. 

 If it is negative the energy expenditure, required to 

 establish in the circuits the corresponding currents, is 

 less than would have been required if they had been 

 isolated. 



An acceleration of the electrons in one circuit pro- 

 duces a retardation of those electrons in an adjacent 

 circuit which are moving in a parallel direction. The 

 interactions of two electrons in motion therefore 

 constitute a stress. In other words, Newton's third 

 law applies to electrons in motion. 



