470     Prof.  W.  H.  Bragg-  and  Mr.  R.  D.  Kleeman  on  the 
bring  out  this  contrast  between  the  effects  at  low  and  at 
high  potentials.  The  figures  are  given  in  the  following 
table  : — 
Relation  of  current  to  potential  gradient  for  different  widths 
of  the  ionization  chamber,  the  currents  being  small  ; 
potential  gradients  in  volts  per  centimetre  :  currents  in 
arbitrary  units,  reduced  to  common  maximum. 
Width  of  Chamber. 
3  mm.  6  mm.  9  mm. 
Potential 
Gradient. 
Current. 
Potential 
Gradient. 
Current. 
Potential 
Gradient. 
Currer 
1000 
400 
1000 
400 
1000 
400 
34-8 
341 
35-7 
335 
36-0 
333 
11-9 
308 
12-7 
302 
16-4 
300 
8-65 
291 
9-47 
292 
9-83 
290 
6-6 
285 
7-47 
283 
7-81 
283 
4-66 
274 
5-50 
272 
5-83 
271 
2-75 
252 
2-75 
247 
3-10 
244 
1-92 
235 
1-89 
221 
2-23 
220 
1-50 
221 
•98 
109 
130 
174 
1-06 
196 
•51 
108 
•84 
134 
•59 
147 
•02 
4 
•04 
68 
•385 
10(5 
•20 
47 
These  figures  are  plotted.,  as  far  as  36  volts  per  cm.,  in 
curves  A,  B,  and  C  of  fig.  1.  An  open  scale  is  chosen  so  as 
to  show  the  separation  of  the  curves  at  low  values  of  the  field, 
when  n  is  not  very  small. 
These  fioures  and  curves  show  that  the  ratio  of  the  current 
at  any  particular  strength  of  field  to  the  saturation  current  is 
almost  independent  of  the  shape  of  the  ionization  chamber, 
when  the  current  is  small.  As  this  seems  an  important  point, 
we  have  made  many  experimental  tests  of  it.  We  give  below 
the  details  of  one  such  test,  in  order  to  illustrate  the  methods 
employed  and  their  degree  of  accuracy. 
The  arrangements  were  the  same  as  those  just  described  ; 
and  the  special  object  of  the  experiment  was  the  deter- 
mination of  the  degree  of  saturation  under  a  certain  moderate 
potential  gradient  in  the  case  of  chambers  of  two  different 
depths.  The  depths  were  reckoned  in  turns  of  the  screw, 
which  raised  the  upper  from  the  lower  plate  of  the  chamber  ; 
eleven  turns  =  1  cm.  The  currents  were  allowed  to  run  into 
the  electrometer  for  10  seconds.  The  electrometer  was  not 
dead-beat,  and  therefore  the  first  and  second  resting-places 
on  the  scale  were  observed,  and  the  mean  taken.  For 
example,  the  second  line,  marked  f,  of  the  subjoined  table, 
