450  Mr.  A.  0.  Rankine  on  the  Decay  of 
the  outer  cylinder  was  about  3  cms.  and  that  of  the  inner  1  cm. 
The  diameter  of  the  wheel  about  which  the  couple  was  applied 
was  about  4"  5  cms. 
Experiments  were  made  with  3*4,  4,  4*2,  4*4,  and  4*5  per 
cent,  solutions,  and  in  each  case  the  couple  was  produced  by 
500  gr.  wt.  acting  round  the  wheel  H.  The  platinum  plate 
was  then  adjusted  so  as  to  come  into  contact  with  the  point  G, 
and  the  action  of  the  valve  beo-an. 
Fio-.  2. 
W\        5  J*>    J    c 
The  friction  of  the  points  C,  E  and  the  pulleys  K,  K  was 
not  large,  being  only  equivalent  to  a  couple  produced  by 
5  gr.  wt.  acting  round  the  wheel  H,  but  in  order  to  obtain  a 
smooth  curve  it  was  necessary  to  eliminate  it.  If  it  is  not 
allowed  for,  the  pen  draws  a  -curve  consisting  of  a  series  of 
alternate  vertical  and  horizontal  lines,  each  upward  step  being 
equivalent  to  the  frictional  couple.  This  difficulty  was  over- 
come in  the  following  way  : — An  iron  plate  S  (fig.  2) 
suspended  from  the  second  arm  of  the  balance  is  adjusted  to 
such  a  distance  from  the  electromagnet  T  that  when  a  given 
current  flows  in  T,  8  is  pulled  downwards  with  a  force  equal 
to  the  frictional  force.  This  causes  the  tension  in  the  strings 
to  be  relieved  to  that  extent.  The  current  flows  in  T 
whenever    the    water    valve    opens.       Hence    the  platinum 
