74  Mr.  Veley  on  an  Apparatus  for  Determination 
Herein,  if  C  nnd  C  are  two  condensers,  R  and  R'  two 
capacity  free  resistances,  then  a  balance  is  attained  when 
R:  R'  =  C  :  C (3) 
The  general  method  of!  working  has  been  to  keep  one 
resistance  and  one  capacity  constant,  and  to  vary  the  other 
resistance  concomitantly  with  the  other  capacity  so  as  to 
restore  the  balance. 
The  method  to  be  described  differs  only  from  the  above  in 
keeping  both  resistances  constant,  and  varying  one  capacity 
concomitantly  with  the  other,  as  also  to  measure  the  capacities 
in  terms  of  a  length  according  to  the  principle  that  cwteris 
paribus  the  capacity  of  a  condenser  varies  inversely  with  the 
distance  between  the  metallic  plates,  provided  that  such 
distance  is  small  relatively  to  the  area  of  the  plates. 
Before  passing  on  to  a  description  of  the  apparatus,  it 
seems  desirable  to  discuss  the  method  worked  out  by  Nernst 
[of.  supra)  and  certain  of  his  pupils  in  Germany,  and  in  this 
country  by  Sir  James  Dewar  and  Prof.  Fleming*.  The  disad- 
vantage of  this  method  appears  to  consist  in  the  fact  that  the 
dielectric  constant  is  determined  indirectly,  since  an  air-con- 
denser is  not  balanced  simultaneously  against  a  condenser 
containing  the  liquid  under  investigation  ;  on  the  other 
hand,  its  advantages  are  (1)  the  substitution  of  a  small 
induction-coil  instead  of  a  rapidly  alternating  commutator, 
and  (2)  the  substitution  of  liquid  resistances  instead  of 
bobbins. 
As  regards  the  latter  improvement,  I  might  mention  that 
for  some  of  my  earlier  experiments  Herr  Wolff  of  Berlin 
v^ound  for  me  two  10,000  (international)  ohm  coils  of  man- 
ganin  according  to  Chaperon's  method  ;  these  coils  were 
placed  in  the  same  box  and  provided  with  connecting  screws 
so  that  either  one  coil  could  be  placed  in  each  of  the  two 
arms  of  the  bridge,  or  both  coils  could  be  put  into  the  same 
arm.  But  these  coils  wTere  discarded  as  self-induction  was 
not  avoided,  and  consequently  it  wras  impossible  to  determine 
the  point  of  minimum  sound  of  the  telephone  owing  to  the 
consequent  and  irritating  after-tone. 
However,  by  using  liquid  resistances  the  above  difficulty 
wras  overcome.  My  apparatus,  as  stated  above,  consisted  of 
two  condensers,  twro  liquid  resistances,  telephone,  and  induc- 
tion-coil with  actuating  battery  ;  the  only  point  of  novelty 
consists  of  the  first  named,  while  of  the  remainder  various 
forms  were  tried  with  a  view  of  obtaining  more  satisfactory 
results. 
*  Proc.  Roy.  Soc.  lxii.  p.  250  (1898). 
