From  §§  2-5,  V.,  it  follows  that  the  permittivity  depends  to
some  extent  upon  the  stresses  in  the  dielectric,  which  may  be
produced  wholly  by  electrical  causes,  unless  the  coefficients  there
defined  vanish.
2.  Method  II.  With  the  Quadrant  Electrometer.  Since  S,  §  5,
III.,  is  proportional  to  c,  0  is  also  proportional  to  c  for  given
values  of  VAy  VBy  and  VAE.  Hence  by  submerging  the  quadrants  in
two  dielectrics  of  permittivities  cl  and  c2  successively  and  measur-
ing the  resulting  deflections  for  the  same  voltages,  we  have

V         'J'l-Wl  |     (0

With  the  Kelvin  Absolute  Electrometer  or  the  Bichat  and  Blond-
lot  Electrometer  In  the  same  way,  by  submerging  the  conduc-
tors of  either  of  these  electrometers  in  two  dielectrics  in  succes-
sion and  measuring  the  corresponding  values  of  Ft  §  4,  III.,  we
have,  for  constant  voltages,

cjc,  =  FJF,  (2)


194         ELEMENTS   OF   ELECTROMAGNETIC   THEORY.
In  the  case  of  the  Kelvin  electrometer  the  force  may  be  kept
the  same  in  the  two  experiments  and  the  comparison  made  by
altering  d.  In  this  case,  if  d^  and  d2  denote  the  values  of  d  when
the  first  and  second  dielectrics  are  in  the  field

,1;;JV:"::,/,    -:.:,         ',/',- W         I.Jv^S       (3)

By  (25),  IV.  and  (3),  III.  the  Kelvin  electrometer  may  also  be
used  when  one  of  the  dielectrics  is  in  the  form  of  a  plane  slab  of
a  given  thickness  dz(<  d).  The  equations  corresponding  to  (4),
III.  and  (5),  III.  are  easily  developed  for  this  case.
When  any  one  of  these  instruments  is  used  idiostatically,
alternating  as  well  as  unidirectional  voltages  may  be  applied,
and  the  permittivity  thus  determined  when  the  time  of  electrisa-
tion is  very  short.
