tional  to  the  drop  of  potential  between  the  two
points  of  the  wire  touched.  If,  now,  the  terminals
of  a  standard  cell  be  inserted  in  tne  circuit  of
the  galvanometer,  so  as  to  oppose  the  current
taken  from  the  potentiometer  wire,  and  the  con-
tacts of  the  potentiometer  wire  be  slid  along  the
wire  until  no  deflection  of  the  galvanometer  needle
is  produced,  the  drop  of  potential  between  these
two  points  on  the  wire  will  be  equal  to  the  differ-
ence of  potential  of  the  standard  cell.  (See  Cell,
Voltaic,  Standard.)

Suppose,  now,  it  be  desired  to  measure  the  dif-
ference of  potential  between  two  points  a  and  b,
on  the  wire  C,  through  which  a  current  is  flow-
ing. Connect  the  points  b  and  d,  and  a  and  c,
as  shown,  with  the  delicate  high  resistance  gal-
vanometer G,  in  either  of  them.  Now  slide  c,
towards  d,  until  the  needle  of  G,  shows  no  deflec-
tion. The  potential  between  a  and  b,  is  then
equal  to  that  between  c  and  d.
Potentiometer  Wire. — (See  Wire,  Po-
tentiometer)

Power. — Rate  of  doing  work.
Mechanical  power  is  generally  measured  in
horse  power,  which  is  equal  to  work  done  at  the
rate  of  550  foot-pounds  per  second.
The  C.  G.  S.  unit  of  power  is  one  erg  per
second.
The  practical  unit  of  power  is  the  watt,  or
10,000.000  ergs  per  second.  The  kilowatt  is
even  more  frequently  used  as  the  unit  of  power
than  the  watt.  (See  Power,  Unit  of.)

Power,  Absorptive The  property


Pow.]


42  L


[Po>v.
possessed  by  many  solid  bodies  of  taking  in
and  condensing  gases  within  their  pores.
Carbon  possesses  marked  absorptive  powers.
