The  Radiation  from  Ordinary  Materials.  207 
The  dimensions  of  the  box  are  32  x  32  x  35  cms. ;  the  front 
side,  which  is  hinged,  is  supposed  to  be  removed  in  order  to 
show  the  interior  arrangements.  A  rectangular  cylinder  of 
coarse  wire  gauze  is  fastened  to  the  ends  A  and  B  of  the 
box,  A  being  covered  with  the  material  under  investigation. 
A  plate  (C)  carrying  the  same  material  could  slide  on  the 
gauze,  always  parallel  to  the  ends.  An  electrode  of  wire 
netting  was  supported,  through  a  slit  in  the  gauze,  inside 
the  wire-gauze  cage  by  a  stiff  wire,  of  which  all  parts  out- 
side the  cage  were  shielded  by  earthed  tubes  (not  shown). 
Now  when  a  large  difference  of  potential  is  established 
between  the  wire  gauze  and  the  electrode,  only  the  ions 
inside  the  cage  can  carry  the  current ;  but  rays  from  outside 
the  cage  can  pass  through  the  interstices  and  ionize  the  air 
inside  as  if  it  were  a  portion  of  an  infinite  volume.  The 
sides  of  the  box  parallel  to  the  wire  gauze  were  8  cms. 
distant  therefrom  and  covered  with  aluminium.  We  shall  see 
that  this  distance  is  sufficient  to  cause  the  total  absorption 
of  the  rays  from  the  edges  of  A  or  C  or  from  the  sides  of 
the  box  before  they  reached  the  cage,  and  so  to  prevent  them 
from  having  any  influence  on  the  ionization  inside  the  cage. 
The  dimensions  of  the  cage  were  18  x  18  X  32  cms. 
Thus,  so  far  as  easily  absorbable  radiation  is  concerned, 
the  ionization  inside  the  wire-gauze  cage  will  be  the  same  as 
that  in  a  volume  of  air  of  identical  dimensions  enclosed 
between  two  infinite  plates  of  the  material  covering  A  and  B, 
for  the  area  of  the  wire  in  the  gauze  is  so  small  that  the  rays 
from  it  produce  no  considerable  effect. 
§  3.  For  the  measurement  of  the  ionization  the  method 
described  in  a  recent  note*  was  adopted.  The  saturation 
current  is  measured  by  balancing  it  against  the  known 
current  through  a  vessel,  containing  uranium,  in  which  the 
pressure  of  the  air  could  be  varied  at  will.  The  capacity  of 
the  compensating  vessel  was  measured  by  comparing  it  with 
that  of  a  small  guard-ring  condenser,  of  which  the  capacity 
could  be  calculated,  by  the  method  described  by  McCleilandt. 
Throughout  the  paper  the  currents  are  expressed  in  ab- 
solute electrostatic  units,  unless  the  contrary  is  express  In- 
stated. 
§  4.  Two  series  of  curves  were  drawn.  In  the  first  the 
box  was  surrounded  by  thick  screens  of  lead  in  order  to  cut 
off  the  external  radiation ;  in  the  second  the  screens  were 
removed.  The  following  metals  were  investigated : — Lead 
(1.  Commercial  sheet),  Lead  (2.  Pure  foil  for  assay);  Copper 
*  Proc.  Camb.  Phil.  Soc,  May  1905. 
t  Hoy.  Dublin  Soc.  Proc.  x.  18,  p.  167. 
