10 
of  Dielectric  Constants  of  Non-conducting  Liquids. 
.Description  of  Condensers. 
These  were  constructed  for  me  by  Mr.  Pje  of  Cambridge, 
according  to  the  specifications  of  the  Rev.  F.  Jervis-Smith, 
F.R.S.,  who  kindly  assisted  me  in  working  out  the  principle 
of  measuring  capacities  and  hence  dielectric  constants  in 
terms  of  lengths. 
Each  condenser  consisted  essentially  of  a  gilt  nickel  dish 
46  mms.  deep  (A  in  sketch)  and  70  mms.  internal  diameter, 
provided  with  a  binding-screw  S  ;  within  each  there  was  a 
moveable  gilt  disk  B  about  58  mms.  diameter  (actually 
59'62  mms.  for  one,  and  57*87  mms.  for  the  other)  and  5  mms. 
thick,  provided  with  a  vertical  metal  spindle  56  mms.  long, 
G,  at  the  end  of  which  was  a  binding-screw  S/  and  a  metallic 
T-piece,  in  the  hollow  of  the  horizont  d  arm  of  which  a  glass 
rod  D  was  cemented  in.  Thi^  form  of  condenser  is  so  fal- 
si milar  to  that  described  by  Cohn  and  Arons*,  except  that 
the  position  of  the  disk  is  variable,  and  the  capacity  of  the 
condenser  thereby  alterable. 
The   following  device   was  adopted  for  moving   the   disk 
vertically  up  and  down,  as  also   for  ensuring  that   its  lower 
surface   remained  strictly  parallel  with  the  upper  surface  ol' 
*  Wied.  Ann,  x.xvui.  p.  4(31  (1886). 
