Cpi ) 



eloclrons will ho <fE - - ff7\. ^Nroi-eoxer wc sliall asci-iho 1(» the 

 elecd'ons a (*(M-laiii kinetic cuei'gT 7\, which Ihcy lia\ (> l)y virtue ot 

 thcii" mass iii tiic ordinary sense ot' the word. Shoidd tlici'c hc no 

 such "true" mass, we have only to put 7'^ =i (). 



One of the tonus that may he uiven to tiie variational equation 

 of motion for a system of material {)articles is 



dö'7\ 

 ÖA = ' — ÖJ\ . 



,Jt 



ö'J\ heiiiu' the change of 7',, if we pass from the real motion to 

 some \arie(l motion in which the \aried |)ositiojis are reached at 

 the same moments as the oriiiinal positions in the i-eal motion, (^ A 

 the virtual work of the forces, and i^' 1\ the im-rement that would 

 he accpiired hy the kinetic eiier<i,y 7\, if variations, equal to the 

 virtnal chan<ies of the cooi-dinates, were im})arte(l to the corresponding 

 velocities (the coordinat(»s themselves being kej)t constant). For our 

 .system of electrons 



(SA z= <]E — öl\\ 



hence, if we nse for (SE the formula (13), 



lit 

 We shall linally mnltii)ly this hy </f and integi-ate from /, to /.,. 

 In case holh the disi)lacemeids q and the \ariations ff^ vanish at 

 the limits, we tind 



.Ji(V 



7' 4 7;)-(r+ i\)\<it = (). 



This is analogous to the principle of least action. 



§ 8. In what precedes there has heen cpiestioji of the \arialiojis 

 of the energies 7' and L\ taken for the system of electrons together 

 with the surrounding aether, which extends 'to infinite distance. 

 Similar though somewhat less sinqtle residts are obtained, if one 

 understands by 7' and (/ the magnetic and the electric energies, in 

 so far only as they belong to the space within an immoxable closed 

 surface o. In \vhal follows ii is lo be understood that this surface 

 may have, ivlatixcly to the system of electrons, any position we 

 like; for simplicity's sake however \ve shall sup|)Ose that it cuts 

 none of them, so that, in exery point (tf <>, the density o =: 0. x\ö 

 to the virtual \ariations, determined by vi and cfo, they need not 

 at all be confined to the part of the system within the surface. We 

 shall denote b\ // ihc noi'inal to the surface, drawn towards the 



