in  a  Discharge  between  Parallel  Plates. 
the 
739 
the  current  when  the  force  is  uniform  ;  so  that  the  potential 
assumed  by  the  wire  ought  to  be  the  same  for  all  small 
currents,  although  it  may  differ  from  the  potential  of  the  gas. 
Fig.  2. — Hydrogen-pressure  1'37  mm. 
250 
200 
150 
100 
50 
5  -^ 
2 
L50 
200 
150 
100 
50 
12  3  4  5  6  7  8 
D/ST/IHCE    OF  W/f?£    f/?OM  W£GAT/l/£   £L£CTRODE  fN  MMS. 
1.  Current  =  1 '210— 6  amperes  per  sq.  cm.  of  electrodes. 
2.  „        6x10-6 
3-          „        3x10-5 
It  would  be  easy  to  make  a  calculation  of  the  charge  in  the 
gas  in  this  case,  and  to  estimate  the  extent  to  which  the 
charge  on  the  ions  disturbs  the  uniformity  of  the  field,  if  the 
velocity  of  the  positive  ion  were  known.  A  rough  estimate 
may  be  made  from  the  numbers  found  for  the  velocity  at 
higher  pressures  and  smaller  forces  by  supposing  that  the 
velocity  is  proportional  to  the  electric  force  and  inversely 
proportional  to  the  pressure.  The  value  thus  found  for  the 
larger  forces  and  smaller  pressures  would  be  a  lower  limit  to 
the  velocity.  It  may  be  conjectured  from  the  determinations 
at  higher  pressures  both  of  the  rate  of  diffusion  and  the 
velocity  under  an  electric  force,  that  the  ions  are  accompanied 
in  their  motion  by  a  group  of  molecules.  If  the  group 
remained  the  same  for  the  lower  pressures  and  higher  forces, 
then  the  velocity  would  be  inversely  as  the  pressure  and  pro- 
portional to  the  electric  force.  It  is  most  probable  that  for 
large  forces  and  small  pressures  the  group  would  diminish  ; 
and  so  the  velocity  under  an  electric  force  would  increase 
more  rapidly  than  the  quotient  -  .     It  may  be  shown  from 
