Flames  containing  Salt  Vapours  for  Alternating  Currents.     489 
Several  different  kinds  of  interrupters  were  tried,  but 
finally  the  ordinary  App's  platinum  contact-breaker  was 
used.  The  platinum  contacts  were  always  carefully  filed 
smooth  before  starting  an  experiment  and  a  10-inch  coil  was 
used  with  a  four-volt  battery,  the  contact-breaker  being- 
adjusted  so  that  the  coil  gave  only  a  short  spark  when  not 
connected  to  the  Leyden  jars.  In  this  way  the  apparatus 
was  made  to  work  sufficiently  steadily  to  obtain  fairly  satis- 
factory observations.  The  distance  between  the  condenser- 
plates  corresponding  to  the  minimum  galvanometer-deflexion 
could  be  obtained  within  about  5  per  cent,  of  its  value.  It 
was  only  after  a  long  series  of  attempts  extending  over  nearly 
a  year  that  the  apparatus  was  got  to  work  well  enough  to 
obtain  reliable  results,  and  numerous  modifications  were  tried 
before  the  form  above  described  was  finally  adopted. 
To  compare  the  conductivities  due  to  different  salts,  the 
increase  in  the  apparent  capacity  of  the  flame-electrodes 
consequent  on  introducing  each  salt  was  calculated  in  terms 
of  the  capacities  of  the  three  condensers. 
With  a  bridge  arrangement  each  arm  of  which  is  a 
capacity  with  negligible  self-induction,  the  condition  for  a 
balance    is    C1C3  =  C2C4,    a    condition   independent   of   the 
frequency.     It  was  found  that  when  the  ratio  ~-  was  altered, 
then  C3  changed  approximately  proportionally,  so  that  it 
appeared  justifiable  to  apply  the  equation  C^Cs^^C^.  Let 
di  be  the  distance  between  the  plates  of  the  adjustable  con- 
denser at  the  minimum  for  the  flame  free  from  salt,  and  d2 
for  the  flame  containing  salt.     The  capacity  of  the  adjustable 
■condenser  in  the  first  case  is  - — -  +  D,  where  D  is  a  quantity 
nearly  independent  of  d±  and  A  the  area  of  each  condenser- 
plate"[01erk  Maxwell,  '  Electricity  and  Magnetism/  art.  202]. 
We  have  therefore 
*.[4+D]= 
CLC 
2V74J 
where  04  is  the  apparent  capacity  of  the  flame-electrodes 
with  the  flame  free  from  salt.  Also  if  (J/  is  the  apparent 
capacity  with  salt  in  the  flame  we  have 
0,i 
Phil.  Mag.  S.  (3.  Vol.  11.  No.  (A.  April  1906.  2  K 
