146  Prof.  W.  Weber  on  Electricity  in  relation  to 
It  may  suffice  merely  to  add  here  a  similar  remark  in  relation 
to  Seebeek's  fundamental  thermomagnetic  experiment.  In  a  body 
which  possesses  the  same  temperature  in  all  its  parts,  the  heat  is 
supposed  to  be  in  a  state  of  mobile  equilibrium ;  or  we  speak,  with 
Fourier,  of  a  reciprocal  radiation  of  the  particles  of  the  body, 
by  virtue  of  which  each  particle  parts  with  just  as  much  heat  to 
the  surrounding  particles  as  it  receives  from  them.  Now,  if 
heat  consists  in  Amperian  molecular  currents,  which,  however, 
are  broken  up  by  the  positive  and  negative  particles  separating 
from  each  other  until  they  encounter  other  particles,  with  which 
they  form  new  molecular  currents,  equilibrium  of  temperature 
must  consist  in  this,  that  the  vis  viva  of  the  electrical  particles 
which  leave  any  part  of  the  body  is  equal  to  the  vis  viva  of  the 
electrical  particles  which  enter  this  part  of  the  body. 
Let  us  now  consider  the  surface  of  contact  of  two  conductors 
which  differ  from  each  other«only  by  greater  masses  of  electricity 
moving  with  smaller  velocity  in  the  Amperian  currents  of  one, 
and  smaller  masses  moving  with  greater  velocity  in  those  of  the 
other.  Then,  when  both  the  conductors  are  at  the  same  tempe- 
rature, the  vis  viva  of  the  electrical  particles  which  pass  from  the 
first  conductor  into  the  second  must  be  equal  to  the  vis  viva 
of  the  electrical  particles  that  pass  from  the  second  conductor 
into  the  first ;  but  the  mass  of  the  electrical  particles  which  pass 
from  the  first  conductor  into  the  second  would  be  greater  than 
the  mass  of  the  electrical  particles  which  pass  out  of  the  second 
conductor  into  the  first.  But  from  this  (if  the  electricity  which 
passes  over  is  always  positive,  while  the  negative  electricity 
remains  behind  in  the  conductor,  to  the  particles  of  which  it 
adheres)  there  would  result  a  difference  of  electrical  charge  on  the 
tivo  sides  of  the  surface  of  contact ;  that  is  to  say,  there  would 
result  an  electromotive  force  at  this  surface  of  contact ;  for  the 
electromotive  force  of  a  surface  of  contact  is  a  force  whereby  a 
difference  of  electrical  charge  is  produced  at  the  two  sides  of 
the  surface  of  contact. 
If  now  the  two  conductors  are  of  such  a  nature  that  this 
difference  of  charge  at  the  two  sides  of  their  surface  of  contact  is 
not  always  the  same,  but  is  greater  or  less  according  to  variations 
of  temperature,  there  would  follow  the  production  of  a  current 
in  a  ring  formed  of  these  two  conductors,  if  different  tempera- 
tures were  to  exist  at  the  two  surfaces  of  contact  of  the  con- 
ductors. 
21.  Helmholtz  on  the  contradiction  between  the  Law  of  Electrical 
Force  and  the  Law  of  the  Conservation  of  Force. 
In   his   memoir,    "Ueber    die     Bewegungsgleichungen    der 
Elektricitat  fur  ruhende  leitende  Korper,"  in  the  Journal  fur 
die  reine  und  angewandte  Mathematik  (vol.  lxxii.  pp.  7  and  8), 
