in  a  Discharge  between  Parallel  Plates.  733 
current  are  generated  in  this  layer  where  the  force  is  large, 
and  the  negative  ions  proceed  from  it  under  a  comparatively 
small  force  along  the  positive  column  to  the  positive  electrode. 
The  layer  near  the  cathode  where  the  ions  are  generated  con- 
tains approximately  the  same  amount  of  gas  as  is  required 
between  parallel  plate  electrodes  to  obtain  the  minimum 
sparking-potential.  The  currents  of  positive  and  negative 
ions  will  be  equal  at  a  point  near  the  end  of  this  layer,  so  that 
a  wire  placed  in  that  position  would  assume  the  true  potential 
of  the  gas.  For  points  very  near  the  negative  electrode,  the 
current  of  positive  ions  exceeds  the  current  of  negative  ions, 
and  the  wire  would  assume  a  potential  higher  than  the 
potential  of  the  gas. 
Along  the  positive  column  the  electric  force  is  compara- 
tively small,  so  that  in  this  portion  of  the  discharge  compa- 
ratively few  molecules  are  ionized.  The  stream  of  negative 
ions  must  therefore  be  in  excess  of  the  positive  stream,  so 
that  in  the  positive  column  the  wire  would  assume  a  potential 
lower  than  the  gas.  As  the  positive  electrode  is  approached 
the  number  of  positive  ions  in  the  current  diminishes  and  the 
number  of  negative  ions  increases,  and  at  the  electrode  the 
current  consists  altogether  of  negative  ions.  When  the  wire 
is  near  the  electrode,  it  would  therefore  assume  a  potential 
much  lower  than  that  of  the  gas.  This  is  a  well-known  expe- 
rimental result;  but  the  above  investigation  shows  that  it 
does  not  follow  that  there  is  any  discontinuity  in  the  electric 
force  near  the  electrode  when  the  wire  is  removed.  There  is 
therefore  no  reliable  evidence  to  show  that  the  so-called 
anode  fall  of  potential  really  exists. 
3.  In  order  to  investigate  experimentally  the  field  of  force 
and  a  current  between  parallel  plates  the  apparatus  which  is 
shown  in  fig.  1  (p.  734)  was  used.  The  current  passed  between 
the  two  zinc  plates  A  and  B.  The  upper  plate  B  was  fixed  to 
a  micrometer-screw  S  which  passed  through  the  brass  plate  C, 
and  in  order  that  the  screw  should  not  shake  in  its  bearing 
the  upper  portion  was  reduced  so  as  to  fit  exactly  into  a  hole 
in  the  cock-piece  F.  The  plate  G  was  fixed  firmly  to  the 
lower  plate  A  by  means  of  four  ebonite  pillars  E.  The  axle  S 
carried  a  divided  circle  D,  so  that  the  distance  between  the 
plates  A  and  B  could  be  determined  accurately.  By  means 
of  the  arrangement  shown  in  the  figure,  it  was  possible  to 
move  the  tine  wire  W  to  any  position  between  the  electrodes. 
The  wire  was  stretched  between  the  ends  of  two  ebonite  rods 
R  which  passed  through  holes  in  the  brass  plate  C.  The 
split  brass  tubes  T  were  fixed  to  the  brass  plate,  and  fitted 
over  the  ebonite  rods,  so  that  the  latter  moved  up  and  down 
