Force  in  the  Contact  of  Metals,  275 
warmer  point  of  contact,  the  thermoelectric  current  always  fol- 
lowed the  same  direction  as  the  current  produced  by  the  electro- 
motive force  of  the  point  of  contact.  This  force  was  conse- 
quently more  intense  at  the  warmer  than  at  the  cooler  point  of 
contact ;  or,  in  other  terms,  the  electromotive  force  of  contact 
increased  with  the  temperature.  These  experiments  took  place 
between  limits  of  temperature  of  about  +10  and  4-20  degrees. 
In  the  experiments  executed  for  the  purpose  of  ascertaining  the 
change  of  temperature  at  the  point  of  contact  on  the  passage  of 
a  current,  the  temperature  of  the  wires  did  not  exceed  +30 
degrees,  and  in  most  cases  was  much  lower.  We  are  therefore 
authorized  to  formulate  the  following  proposition  : — 
3.  The  electromotive  force  of  contact,  for  the  eleven  metallic 
combinations  investigated,  increased  with  the  temperature  when  the 
experiments  took  place  at  a  temperature  not  exceeding  +  30  degrees. 
That  this  proposition  cannot  be  applied  to  all  temperatures 
results  from  the  researches,  on  thermoelectric  phenomena,  which 
several  men  of  science  have  executed  at  higher  temperatures. 
From  these  it  is  known  that  the  thermoelectric  current  may  di- 
minish with  the  increase  of  the  temperature  at  the  points  of 
contact.  From  the  numbers  given  for  the  two  series,  it  follows 
that  the  thermoelectric  current  is  greater  for  the  metallic  com- 
binations whose  electromotive  force  is  greater  than  for  those  pos- 
sessing an  inferior  electromotive  force.  The  exact  ratio  between 
the  electromotive  forces  and  the  corresponding  thermoelectric 
currents  is  obtained  by  dividing  the  numbers  of  one  of  the  series 
by  the  corresponding  numbers  of  the  other.  The  following 
Table  gives  the  quotients  obtained  by  dividing  the  numbers  de- 
signating the  amounts  of  the  electromotive  force  by  those  repre- 
senting the  magnitudes  of  the  corresponding  thermoelectric 
currents : — 
Iron-copper    .     . 
Cadmium-copper 
1-12 
1-42 
Copper-tin  .  .  1*57 
„       -aluminium  1*37 
Zinc-copper    .     . 
Copper- silver  .     . 
„      -gold    .     . 
224 
1-47 
1-62 
„  -platinum  1*30 
„  -palladium  1*20 
„      -bismuth      1*97 
„      -lead    .     . 
1-23 
Relying  on  theory,  one  would  have  expected  that  these  quo- 
tients would  be  equal  in  amount,  or  (which  comes  to  the  same 
thing)  that  the  electromotive  and  thermoelectric  forces  would  be 
proportional  to  each  other ;  but  this  is  far  from  being  the 
case.  As  a  rule,  the  quotients  diminish  in  proportion  as  the 
electromotive  forces  increase.  It  is,  in  my  opinion,  certain 
that  this  departure  from  theory  cannot  depend  on  possible 
errors  of  observation.     Doubtless  these    errors,  in  the   deter- 
T2 
