Flames  containing  Salt  Vapours  for  Alternating  Currents.    485 
conductivity  with  the  amount  of  salt  present  in  the  flame 
and  with  the  nature  of  the  salt  was  investigated.  The 
variation  of  the  conductivity  with  the  frequency  of  alter- 
nation, the  maximum  electromotive  force,  and  the  distance 
between  the  electrodes  was  also  examined.  The  results 
obtained  enable  a  comparison  to  be  made  between  the  con- 
ductivities of  the  various  alkali  salt  vapours  for  alternating 
currents  and  their  conductivities  for  steady  currents  as 
previously  determined  *. 
It  appears  that  the  relative  conductivities  for  rapidly 
alternating  currents  are  nearly  proportional  to  the  square 
roots  of  the  corresponding  conductivities  for  steady  currents. 
The  flame  is  found  to  behave  for  very  rapidly  alternating 
currents  more  like  a  dielectric  of  high  specific  inductive 
capacity  than  like  a  conducting  medium  ;  and  it  is  shown 
that  this  result  is  in  accordance  with  the  ionic  theory. 
The  rest  of  the  paper  is  divided  into  the  following 
sections  : — 
(1)  Description  of  apparatus  used. 
(2)  Variation  of  the  conductivity  with  the  concentration 
and  nature  of  the  salt  vapour. 
(3)  Variation    of    the    conductivity    with    the    maximum 
P.D.,  the  frequency,  and  the  distance  between  the 
electrodes. 
(4)  Theory    of  the    conductivity  for    rapidly  alternating 
currents. 
(5)  Summary  of  results. 
(1)  Description  of  Apparatus  used. 
To  produce  a  steady  flame  containing  a  definite  amount  of 
salt  vapour,  an  apparatus  similar  to  those  described  in  the  two 
papers  referred  to  above  was  used.  The  principal  parts  of 
the  apparatus  are  shown  in  fig.  1. 
A  mixture  of  coal-gas  and  air  containing  spray  of  a  salt 
solution  was  burnt  from  a  glass  tube  TT  tipped  with  a  short 
thin  copper  tube  1  cm.  in  diameter. 
The  mixture  was  formed  in  a  glass  bulb  B,  from  which  it 
passed  through  a  wide  tube  AA  to  the  burner.  In  the  bulb 
and  AA  the  coarser  spray  settled,  and  was  allowed  to  escape 
through  the  tubes  D  and  DV    The  spray  was  produced   by  a 
*  "  The  Electrical  Conductivity  and  Luminosity  of  Flames  containing 
Vaporised  Salts,"  bv  A.  Smithells,  II.  M.  Dawson,  and  II.  A.  Wilson, 
Phil.  Trans.  A  193.  1899.  "  On  the  Electrical  Conductivity  of  Flames 
containing-  Salt  Vapours,"  by  II.  A.  Wilson,  Phil.  Trans.  A  192.  1899. 
