744  Prof.  J.  S.  Townsend  on  the  Field  of  Force 
It  would  be  possible,  no  doubt,  to  use  a  very  high  electro- 
motive force  and  a  large  external  resistance,  and  most 
probably  a  steady  current  o£  any  value  could  then  be  obtained, 
but  for  a  preliminary  investigation  it  was  not  considered 
necessary  to  make  experiments  in  that  direction. 
Experiments  were  also  made  at  higher  pressures,  but 
owing  to  continued  use  of  the  apparatus  the  electrodes 
underwent  some  change  and  the  glow  did  not  spread 
uniformly  over  the  surface  of  the  electrodes.  It  frequently 
happened  that  the  current  passed  through  a  narrow  column 
of  gas,  so  that  it  was  impossible  to  make  observations  of 
potential  or  to  determine  the  density  of  the  current  per 
square  centimetre  of  the  electrodes.  From  a  theoretical 
point  of  view  it  is  better  to  make  experiments  at  the  lower 
pressures  (2  or  3  times  the  critical  pressure)  in  order  to 
obtain  experimental  evidence  of  the  transformation  of  the 
electric  field,  as  the  potential  of  the  wire  does  not  differ  so 
much  from  that  of  the  gas  at  these  pressures  for  the  smaller 
currents.  It  may  be  seen  from  the  numbers  that  have  been 
given  for  the  ratio  of  the  currents  of  positive  and  negative 
ions  that  the  number  of  negative  ions  in  the  gas  near  the 
negative  electrode  diminishes  as  the  pressure  rises. 
7.  Another  set  of  experiments  of  a  different  kind  have 
been  made  to  test  the  results  which  are  indicated  by 
theoretical  considerations.  It  has  been  shown  that  the 
sparking  potential  is  the  same  as  the  potential  required  to 
maintain  a  small  current,  but  as  the  field  of  force  near  the 
negative  electrode  is  increased  by  the  positive  charge  in 
the  gas,  the  potential  required  to  maintain  a  large  current 
diminishes.  If  this  theory  is  correct,  it  should  be  possible 
to  produce  sparking  or  to  maintain  a  small  current  between 
parallel  plate-electrodes,  if  the  force  near  the  negative 
electrode  is  increased  by  having  a  positively  charged  body 
near  it.  This  can  easily  be  arranged  by  having  a  grating 
of  fine  wires  near  the  negative  electrode  and  charging  it  by 
means  of  a  battery  or  a  small  replenisher. 
A  simple  form  of  apparatus  to  illustrate  this  effect  is 
shown  in  fig.  4.  A  pair  of  plane  parallel  electrodes  N  and 
P  were  fitted  in  a  glass  tube  of  about  2\  centimetres  in 
diameter.  A  grating  of  fine  wires  was  mounted  on  a  brass 
ring  R  which  was  fixed  to  the  end  of  an  ebonite  tube  T.  The 
ebonite  tube  was  shaped  so  that  the  ring  fitted  over  the  end 
and  the  wires  of  the  gauze  rested  on  the  front  of  the  ebonite 
that  projected  into  the  ring.  The  negative  electrode  fitted 
inside  the  ebonite  tube  and  could  be  placed  at  any  convenient 
distance  from  the  grating.     The  wires  of  the  grating  were 
