Ionization  produced  by  a  Rays. 
mi 
The  method  of  Bragg  and  Kleeman,  which  was  so  well 
adapted  to  the  determination  o£  the  ionization  per  cm.  along 
the  early  portion  of  the  path  of  the  a.  particle,  did  not  seem 
to  be  equally  well  adapted  to  the  present  problem.  In  their 
method,  it  is  necessary  to  use  a  pencil  of  rays  of  small  angle  and 
a  very  narrow  testing-vessel;  conditions  which  it  is  practically 
impossible  to  employ  in  the  measurement  of  a  very  small 
ionizing  action,  such  as  it  was  thought  the  a  particle  might 
still  produce  after  passing  through  7  cms.  of  air. 
Fig.  1. 
To  Electrometer. 
Active  Wire  — 
I  T 
MlCfi    or  Al. 
es»t^» 
To  Battery 
'"*— >  To  Fa mp 
and    Gauge 
The  method  therefore  adopted  in  the  present  experiment 
was  always  to  measure  the  total  ionization  caused  by  the 
a  particle  beyond  a  certain  distance  from  the  source.  Fig.  1 
shows  the  arrangement  of  the  testing-vessel  employed.  T, 
the   testing-vessel   proper,  is   a  brass   cylinder  of  sufficient 
