The  two  rods  of  the  thermo-element  are  of  different  elec-
trical conductivity.  If,  therefore,  the  ends  of  the  rods  at
the  hot  junction  are  heated,  a  difference  of  potential  is  pro-
duced, causing  an  electric  current  to  flow,  varying  in  strength
with  the  degree  of  the  thermal  difference  between  the  cold
and  the  hot  junctions,  or  with  the  intensity  of  the  heat  to
which  the  thermo-element  is  exposed.  The  relation  of  this
current  to  the  temperature  has  been  determined  accurately
by  experiment,  and  the  scale  of  the  galvanometer  can  there-,
fore  be  divided  to  read  directly  in  Fahrenheit  or  Centigrade
degrees.  Thus,  as  soon  as  the  thermo-element  is  exposed
to  heat  or  cold,  the  electric  pressure  or  current  produced  in
the  two  rods  actuates  the  mechanism  of  the  galvanometer,
and  the  needle  of  the  latter  indicates  directly  the  exact


182  APPLIED  SCIENCE

temperature  of  the  hot  junction  at  the  place  where  the
thermo-element  is  inserted.
Inasmuch  as  the  electric  current  produced  in  the  thermo-ele-
ment through  the  heating  of  the  hot  junction  depends  on  the
difference  between  the  temperature  at  the  two  extremes  of
the  rods,  it  is,  of  course,  essential  that  the  outer  ends  of  the
rods  or  the  cold  junction  be  kept  cool.
