the  negative  and  positive  plates  of  2,  the  final  voltage  being  Vv
Immediately  after  charging

ql  =  SL  V    and     q2  =  S2F
After  the  final  connection  in  multiple

9t-9t-(Sl-SJV=(Sl  +  S1)ri
Hence

S*ISi  =  (V-V2)l(V+V2)  (19)

It  is  obvious  that  the  above  three  methods  cannot  be  applied
when  one  or  both  of  the  condensers  are  of  the  guard  ring  type,
thus  having  more  than  two  conductors.  The  following  method  of
testing  the  equality  of  the  capacities  of  two  guard  ring  condensers
was  devised  by  Maxwell.  It  can  also  be  applied  when  only  one,
or  neither,  of  the  condensers  is  of  the  guard  ring  type.  In
the  last  case  it  becomes  identical  with  the  last  of  the  preced-
ing methods,  which  is  an  extension  of  a  method  due  to  Caven-
dish.
10.  Maxwell's  Method  of  Testing  the  Equality  of  the  Capacity
of  a  Guard  Ring  Condenser  and  that  of  any  Other  Condenser.*  Let
A  be  the  disk,  B  the  guard  ring  and  sheath,  and  C  the  larger
plate  of  one  of  the  condensers  ;  and  let  A' ',  B' ,  and  C'  be  the
corresponding  parts  of  the  other.  If  either  condenser,  as  ABC,
is  of  the  simpler  form  with  only  two  conductors,  we  have  only  to
suppress  B  and  to  suppose  A  and  C  to  be  the  two  conduc-
tors, it  being  understood  that  sensibly  all  the  tubes  of  induc-
tion pass  from  one  plate  to  the  other  when  the  condenser  is
charged.
Let  B  be  kept  always  connected  with  C' ,  and  B'  with  C.     Then

(i)  Let  A  be  connected  with  B,  and  C'  with/,  the  positive

(for  the  sake  of  definiteness)  terminal  of  a  battery  or  other  source

of  electrification,  the  other  terminal  of  which  is  connected  to

*  Maxwell,   Treatise,  §  229.
