476     Prof.  W.  H.  Bragg  and  Mr.  R.  1).  Kleeman  on  the 
gradients,  and  this  is  in  accordance  with  the  present  hypo- 
thesis. For.  when  the  currents  are  so  large,  the  value  of  xrr 
is  great,  and  the  effects  of  general  recombination  must  be 
considerable,  unless  the  potential  gradient  is  much  increased. 
In  fact,  the  general  characteristic  of  these  curves  is  that  the 
larger  the  current  the  higher  the  potential  gradient  must  be 
at  the  point  where  the  effects  of  altering  the  depth  of  the 
chamber  cease  to  be  visible. 
Several  observers  have  determined  the  form  of  the  curve 
connecting  current  and  potential  gradient,  and  have  calcu- 
lated therefrom  the  recombination  coefficient.  Let  us  now 
consider  the  result  of  neglecting  the  effects  of  initial  recom- 
bination  in  these  calculations. 
If  the  currents  are  very  great,  the  effects  of  initial  recom- 
bination may  be  small  as  compared  with  those  of  general 
recombination.  But  they  must  always  be  there,  and  their 
effect  will  be  of  greater  relative  importance  when  the  current 
is  made  smaller,  either  by  using  a  weaker  source  of  ionization 
or  by  lessening  the  width  of  the  ionization  chamber.  If  both 
effects  are  ascribed  to  one  cause,  whose  influence  is  measured 
by  a,  then  a  must  be  given  a  value  which  is  fictitiously 
large.  The  smaller  the  chamber,  the  greater  the  apparent 
value  of  a  must  be;  and  this  is  actually  the  case,  as  found 
and  remarked  upon  by  Retschinsky.  For  when  the  chamber 
is  verv  small  the  effects  of  general  recombination  ought  to  be 
small  :  and  if,  as  is  the  case,  there  is  still  a  considerable  lack 
of  saturation  at  moderate  voltages,  the  whole  of  which  is 
ascribed  to  general  recombination,  the  value  found  for  a 
must  be  very  great.  It  is  possible  to  find  any  desired  value 
of  a  in  this  way,  if  only  the  currents  are  made  small  enough. 
This  is  especially  true  if  we  use  the  first  formula  employed 
by  Retschinsky,  and  ascribed  by  him  to  liiecke.  In  this  the 
determination  of  «  depends  on  the  difference  between  two 
current- values  taken  from  the  upper  part  of  the  curve,  where 
the  slope  is  due  rather  to  initial  than  to  general  recombination. 
To  make  this  point  clear  consider  the  following  determinations 
of  a  : — 
Retschinsky  gives  the  following  form  of  Riecke's  equation 
where  the  quantities  are  expressed  in  electrostatic  unit-  : — 
a  =  5*52  X  10 
iTM1--**—} 
where  C  =  saturation  current  per  sq.cm.  of  electrode, 
c  =  current  for  a  potential  gradient  F0, 
/  =  depth  of  chamber. 
