in  a  Discharge  between  Parallel  Plates.  743 
change  occurs  in  the  field  of  force,  and  a  cathode-fall  of 
potential  of  about  240  volts  is  developed  in  the  layer 
4  millimetres  thick  near  the  cathode.  The  curve  shows  that 
the  potential  between  the  electrodes  required  to  maintain  the 
current  diminishes  when  the  cathode-fall  of  potential  is 
developed. 
300 
Fig.  3.- 
-Hydrogen-pressure  2*74  millimetres. 
300 
/    1 
a 
s^^** 
200 
■ 
/ 
s"^ 
\j 
100 
/ 
/ 
/ 
zoo 
100 
!  2  3  4  5  6  7  Q 
D/ST/IMC£    OF  W/f?£    £f?OM  N£GAT/V£  £l£CT/?0£>£  /N  MMS. 
1.  Currents  less  than  2xl0-6  ampere  per  sq.  cm. 
2.  Current=4x  10~6  ampere  per  sq.  cm. 
The  calculations  which  have  been  given  for  the  distribution 
of  the  current  between  positive  and  negative  ions  show  that 
at  a  point  between  6  and  7  millimetres  from  the  negative 
electrode,  the  two  streams  are  equal  over  a  range  of  pressures 
between  2  and  9*27  millimetres,  and  the  wire  should  take 
up  a  potential  nearly  equal  to  the  potential  of  the  gas  at 
those  points.  The  latter  potential  would  be  represented  by 
a  straight  line  joining  the  two  ends  of  the  curve  for  the 
smaller  currents,  and  the  figures  show  that  these  lines  would 
intersect  the  potential  curves  determined  by  the  wire  at 
points  about  6  millimetres  from  the  negative  electrode. 
It  is  not  easy  to  find  other  potential  curves  intermediate 
between  those  given  in  the  figure  as  the  current  is  unsteady 
for  values  between  2  x  10-6  and  4  x  10 ~6  ampere,  where  the 
charge  in  the  gas  exercises  an  appreciable  effect  on  the  field m 
