732  Prof.  J.  S.  Townsend  on  tlie  Field  of  Force 
in  the  gas  2  millimetres,   for  instance,    from  the  negative 
electrode  would  be  —  when  the  current  is  small. 
4 
The  question  then  arises,  whether  a  fine  wire  placed  at  a 
distance  of  2  millimetres  from  the  negative  electrode  would 
assume  a  potential  of  — .     It  is  easy  to  see  that  if  the  wire 
is  brought  to   a   potential  -j   and  then  insulated,  it  would 
rapidly  acquire  a  higher  potential.  Considering  the  case  in 
which  the  hydrogen  is  at  a  pressure  of  two  millimetres,  the 
calculations  which  have  been  made  show  that  when  the  field 
V 
is  undisturbed,  the  wire  being  at  potential  -r ,  the  number  of 
negative  ions  passing  the  wire  per  second  is  less  than 
4  per  cent,  of  the  number  of  positive  ions.  The  number  of 
positive  ions  discharged  on  the  wire  would  be  largely  in 
excess  of  the  number  of  negative  ions ;  it  would  therefore 
acquire    a    positive    charge,    and    its    potential  would   rise 
above  — .     The  positive  charge  on  the  wire  would  tend  to 
repel  positive  ions  and  attract  negative  ions  until  the  numbers 
of  each  kind  acquired  per  second  become  equal.  It  would 
be  almost  impossible,  owing  to  the  mathematical  difficulties, 
to  calculate  the  amount  to  which  the  potential  would  rise  ; 
but  it  would  be  undoubtedly  very  considerable.  A  com- 
parison between  the  tables  given  for  the  two  pressures  shows 
that  the  discrepancy  between  the  potential  of  the  wire  and 
the  undisturbed  potential  of  the  gas  increases  as  the  pressure 
rises. 
It  will  be  noticed  that  between  6  and  7  millimetres  from 
the  negative  electrode  the  positive  and  negative  currents  are 
equal,  so  that  in  that  neighbourhood  the  wire  would  most 
probably  take  up  the  potential  of  the  gas. 
It  may  also  be  seen  that  the  wire  would  assume  a  potential 
lower  than  that  of  the  gas  when  placed  near  the  positive 
electrode. 
2.  It  is  interesting  to  examine  on  these  principles  what 
might  be  expected  with  pressures  above  the  critical  pressure 
when  a  large  current  is  flowing,  and  the  number  of  ions  in 
the  gas  produce  an  appreciable  alteration  in  the  distribution 
of  electric  force  between  the  electrodes.  According  to  the 
theory,  the  charge  in  the  gas  would  alter  the  field  of  force  so 
as  to  give  rise  to  a  fall  of  potential,  across  a  layer  of  gas  near 
the  negative  electrode,  of  the  same  order  as  the  minimum 
sparking-potential.     Nearly  all  the  ions  which  constitute  the 
