PHYSICAL CONCEPTION OF THE VELOCITY a. 599 



In order to have the total action of the first plane on the 

 movable portion in question bl of the second, we must integrate 

 this expression from x= - oo to #=+co, which gives 



+co 



=2 ~~ 



or 





On the other hand, the electrostatic charge of this surface is 

 bl<r'. As the action of the former unlimited plane on the unit of 

 mass is equal to 27ro-, the repulsion /' exerted on this surface is 

 perpendicular, and its value is 



/' = 2ir<r<r'bl. 



If these two actions are equal, the same would be the case 

 for all other portions of the second plane, and there would be 

 equilibrium between them. For this uu' must be equal to # 2 , or, 

 if the velocities u and u' are equal, u = a. 



The constant a is therefore such that if two unlimited parallel 

 planes, uniformly electrified, moved in the same direction with 

 this velocity, their electrodynamic action would counterbalance 

 their electrostatic repulsion. As the velocity a is that of light, the 

 experiment cannot be realised in this form. 



617. In order to evaluate the order of magnitude of the effects 

 which may be obtained, we may observe that an unlimited band 

 of breadth ft, and density o-, movable in its own direction with a 

 velocity u, is equivalent to a current whose electromagnetic in- 



tensity is b. If we suppose it placed at a distance e from a 



similar band, and if the condenser thus formed be charged to an 

 electrostatic potential V, we shall have (74) 



Now, by means of electrical machines, we can get potentials 

 equal to 100,000 Daniell's cells that is to say about io 5 volts, or 



