ISDEXEB 
THE 
LONDON,  EDINBURGH,  and  DUBLIN 
PHILOSOPHICAL    MAGAZINE 
AND 
JOURNAL    OF    SCIENCE. 
[SIXTH   SERIES.] 
JUNE    1906. 
LXVII.   The  Field  of  Force  in  a  Discharge  between 
Plates.     By  J.  S.  Townsend,  M.A.,  F.R.S.* 
1.  T^ROM  the  theory  which  has  been  given  by  the  author  f 
JF  to  explain  the  large  difference  between  the  sparking- 
potential  and  the  potential  required  to  maintain  a  discharge, 
it  is  to  be  expected  that  large  variations  in  the  electric  field 
accompany  changes  in  the  current  between  two  parallel 
plates  in  a  gas.  A  number  of  experiments  have  been  made 
with  a  view  to  obtaining  evidence  as  to  the  nature  of  these 
changes.  The  results  are  in  good  accordance  with  the 
general  indications  given  by  the  theory,  but,  owing  to  im- 
perfections in  the  experimental  methods  which  are  employed, 
it  is  impossible  to  obtain  accurate  determinations  of  the 
electric  field. 
The  only  practical  method  that  has  been  employed  to 
determine  the  potentials  at  various  points  of  the  discharge  is 
by  means  of  a  wire  placed  in  the  path  of  the  discharge.  It 
has  generally  been  assumed  that  the  wire  does  not  disturb 
the  potential  in  the  discharge,  and  when  the  gas  is  conducting 
the  wire  is  supposed  to  acquire  the  potential  at  any  point 
which  would  exist  there  if  the  wire  were  removed.  It  is 
difficult  to  see  why  this  should  be  universally  the  case  ;  and 
on  consideration  it  would  appear  that  when  the  wire  is  near 
one  of  the  electrodes,  the  potential-fall  between  the  electrode 
and  the  wire  is  probably  much  greater  than  the  potential-fall 
*  Communicated  by  the  Author. 
t  Philosophical  Magazine,  March  1005. 
Phil.  Mag.  S.  6.  Vol.  11,  No.  66.  June  1906.  3  13 
