in  a  Discharge  betiveen  Parallel  Plates, 
731 
It  is  interesting  to  'consider  the  cases  for  two  different 
pressures  above  the  critical  pressure. 
Thus  let  the  plates  be  8  millimetres  apart,  and  let  the  gas 
be  hydrogen  at  2  millimetres  pressure.  In  this  case  the 
sparking-potential  is  282  volts,  which  is  also  the  potential 
-%• 
required  to  maintain  a  small  current.     The  value  of  —  will 
P       R 
therefore  be  176,  and  the  corresponding  values  of  -  and  — 
1  p  p 
are  2*41  and '055  respectively*;  so  that  a  =  4V83  and/3  =  *110. 
The  following  are  the  values  of  the  ratio  —  corresponding 
to  different  distances  from  the  negative  electrode: — 
X    ... 
0. 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
n2v 
0 
0143 
•038 
•078 
•150 
•287 
•59 
1-58 
OO 
It  may  thus  be  seen  that  for  the  greater  part  of  the  distance 
between  the  plates  the  current  consists  principally  of  a  stream 
of  positive  ions,  since  the  ratio  —  is  less  than  unity  for  more 
than  6  millimetres  of  the  distance.  At  a  point  between  6  and 
7  millimetres  from  the  negative  electrode,  the  numbers  of 
positive  and  negative  ions  passing  unit  section  are  equal,  and 
from  that  point  up  to  the  positive  electrode  the  current  carried 
by  the  negative  ions  exceeds  the  current  carried  by  the 
positive  ions. 
It  is  interesting  to  see  how  the  numbers  change  as  the 
pressure  and  sparking-potential  rise.  As  before,  let  the 
plates  be  8  millimetres  apart  and  the  pressure  9*27  millimetres 
instead  of  2  millimetres,  In  this  case  the  sparking-potential 
is  487  volts,  and  the  values  of  a.  and  ft  are  6'0'6  and  -05 
respectively.  The  corresponding  ratios  for  various  values  of 
the  distance  x  are  : — 
X  ... 
0. 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8. 
n2v 
0 
•007 
•020 
•044 
•090 
•190 
•502 
1-20 
OO 
If  the  negative  electrode  be  taken  as  at  zero  potential  and 
the  positive  electrode  at  potential  Y,  the  potential  at  a  point 
*  J.  S.  Townsend,  Phil.  Mag.  Nov.  1903. 
3132 
