WILSON AND LKWIS. — RKLATIVITY 



485 



If then p is the density of the moving ehurge, we must write 



Po 



Vl — t,2" 

 When we eompare the two \ectors 



(130) 



eW = 



(v + k^) and p^,w = p (v + k.,) 



Vl — r2 

 with the two vectors which we have obtained for a material system 



7»oW = ni (v + k4) 



and 



MiiW 



Mo 



Vl 



(v + k.,) 



we see that they are identical in mathematical form. But the com- 

 ponents of ew are not quantities which are conmionly used in physics, 

 while the components of poW are 

 the density of electricity and of 

 electric current. On the other 

 hand the components of ///oW are 

 the fundamental quantities known 

 as mass and momentum, while the 

 components of /xqW are not com- 

 monly used. This is probably due 

 to the fact that the fundamental 

 conservation law for electricity is 

 26 = const., whereas the funda- 

 mental conservation law for mass 

 is not Zwo = const., but 2w = 

 const. 



55. We may now construct the 

 potential at a point due to a con- 

 tinuous distribution of electricitv, 

 directly from (91) and (127). 



Figure 28. 



m = / (r/Sxq)* ^• 



(131) 



The interpretation of this equation will be evident from an examination 

 of a diagram which is an inunediate extension of the one previously 

 used in discussing potentials. And we may then show that, when a 

 particular space and time have been assumed, the components of the 



