Recombination  of  Ions  in  Air  and  other  Gases.        467 
this  is  done,  it  is  found  that  it  is  still  necessary  to  apply  a 
high  potential  in  order  to  extract  all  the  ions  from  the  gas. 
For  example,  in  air  at  atmospheric  pressure  an  electric  force 
of  25  volts  to  the  cm.  will  only  extract  about  80  per  cent,  of 
the  ions  which  are  obtained  when  the  force  is  increased  to 
1000.  The  following  example  will  serve  as  an  illustration: — 
The  width  of  the  ionization  chamber  is  4  mm.,  the  upper 
electrode  being-  a  metal  plate,  the  lower  a  sheet  of  gauze.     A 
or  o 
thin  layer  of  radium  is  placed  (j"2  cm.  below  the  sheet,  and  a. 
particles  emitted  from  Ha  C  cross  the  chamber  and  ionize  the 
air,  which  is  at  atmospheric  pressure.  The  area  of  the  plate 
on  which  the  rays  fall  is  about  18  sq.  cm.  The  capacity  of  the 
electrometer  to  which  the  upper  plate  is  connected  is  about 
150  cm.,  and  a  potential  of  *125  volt  applied  direct  to  the 
electrometer  causes  a  deflexion  of  722  divisions  on  the  scale ; 
ten  divisions  =  1  mm.  When  the  lower  plate  is  raised  to 
400  volts  positive,  so  that  the  electric  force  is  1000  volts 
per  cm.,  there  is  a  deflexion  of  982  divisions  in  10  seconds, 
under  the  influence  of  the  a  rays.  When  a  potential  of  10 
volts  is  applied,  giving  a  force  of  25  volts  per  cm.,  there  is  a 
deflexion  of  772  in  10  seconds. 
In  the  latter  case  the  charge  Q  received  per  sq.cm.  of 
electrode  in  one  second,  measured  in  electrostatic  units,  is — 
772x150  0  „      in    . 
==  A'  \  v    |()— ^ 
10x722x8x300x18  JXLJ    ' 
The  number  of  ions  falling  on  each  sq.cm.  of  electrode  per 
second  is  therefore  1*2  X  10°  nearly. 
The  velocity  of  ions  at  this  potential  gradient  is  nearly 
25  x  1'5,  or  37  cm.  per  second. 
Thus,  if  n  be  the  number  of  ions  in  a  cubic  centimetre, 
3772  =  1-2  x  10G,  and  therefore  n  =  3*2  x  104.  Hence,  the 
number  of  recombinations  taking  place  in  a  second  in 
the  space  between  two  opposing  square  centimetres  of 
the  electrodes  is  equal  to  a  x  *4  x  (3*2  x  104)2.  If  we  take  the 
value  of  a  to  be  3400  X  3  x  10-10,  we  find  this  number  to  be 
nearly  420.  Finally,  therefore,  the  number  of  ions  recom- 
bining  in  each  second  is  420,  whilst  the  number  received  is 
1-2  x  106,  and.  thus  only  1/3000  of  the  ions  are  lost  in  this  way. 
But  the  current  at  25  volts  is  only  772/082,  or  about  80  per 
cent,  of  the  current  at  1000  volts. 
It  is  clear  from  this  example  that  there  is  some  cause  which 
prevents  the  current  attaining  its  full  value  other  than  general 
recombination  between  positive  and  negative  ions. 
Now,  it  is  possible  that  ions  newly  formed  might  he 
specially  liable  to  recombine  with  each  other.     Such  a  pos<i- 
