the  Principle  of  the  Consei'vation  of  Energy. 
dr*_ 
dr* 
repel  each  other ;  thus  when  -j-^  <cc  +  2rff  they  repel  only  so  long 
as  r  > -p  ee'j  and,  on  the  contrary,  they  attract  when 
_  2  e  +  e>    ,    ' 
cc     ee 
An  exception  to  this  rule  occurs  only  in  the  case  in  which 
(e  +  e7  ee1  \ 
r—2 -, ),  which  is  always  a  factor  of  the  denominator, 
becomes  likewise  a  factor  of  the  numerator.     This  case  occurs 
when   the   two   electrical   particles   are   at  permanent   relative 
rest,  so  that  -=-  =0  and  75  =0. 
at  dt2 
The  general  expression  for  the  force  given  above  becomes  in 
fact 
\  ee'    cc  J 
(•♦&) 
dv  £  c 
when  —  =0;  and  by  dividing  this  by  the  mass n  we  find  the 
part  of  the  acceleration  which  is  due  to  the  forces  exerted  upon 
each  other  by  the  two  electrical  particles,  namely 
(6  +  eV  (i+zaft 
,  (       <}e+e'    ee'\  \    ^  cc  J J' 
eer(  r  —  2 r  •  —  ) 
\  ee'      cc  / 
By  adding  to  this  the  other  part  of  the  acceleration,  namely  /, 
which  is  due  to  the  acquired  motion  of  the  particles  at  right 
angles  to  r  and  to  the  action  of  other  bodies,  we  obtain  the  total 
acceleration,  namely 
ddr                       (e  +  e^ee1 
W     U       ,  /       9e  +  e' 
ee'rl  r—2 — r 
\            ee' 
ee' 
cc 
(•+§/). 
which,  when  the  particles  are  at  permanent  relative  rest,  =0. 
Hence  for  permanent  relative  rest  we  have 
-         e  +  e7    ee! 
ee7      ri- 
ll this  value  of/  be  substituted  in  the  expression  for  the  force 
\  ee'      cc  J 
K^lh 
