﻿920 
  Prof. 
  Bragg 
  and 
  Mr. 
  Madsen 
  : 
  An 
  Experimental 
  

  

  the 
  chamber-walls, 
  a 
  very 
  large 
  proportion 
  of 
  thd 
  ionization 
  

   produced 
  is 
  actually 
  due 
  to 
  the 
  emergence 
  rays 
  of 
  the 
  plate. 
  

   But 
  if 
  the 
  plate 
  is 
  taken 
  away, 
  the 
  place 
  of 
  these 
  rays 
  is 
  

   supplied, 
  to 
  a 
  varying 
  extent, 
  by 
  secondary 
  rays, 
  made 
  in 
  the 
  

   air 
  or 
  emergent 
  from 
  tbe 
  last 
  substance 
  traversed 
  by 
  the 
  

   rays. 
  A 
  normal 
  stream 
  of 
  7 
  rays 
  always 
  contains 
  rays 
  ; 
  

   if 
  these 
  are 
  stopped 
  by 
  a 
  screen, 
  fresh 
  ft 
  rays 
  emerge 
  from 
  

   the 
  other 
  side 
  of 
  the 
  screen. 
  It 
  is 
  impossible, 
  therefore, 
  to 
  

   measure 
  the 
  emergence 
  rays 
  by 
  subtracting 
  the 
  value 
  of 
  the 
  

   current 
  when 
  the 
  above-mentioned 
  plate 
  is 
  not 
  in 
  position 
  from 
  

   the 
  value 
  when 
  it 
  forms 
  part 
  of 
  the 
  chamber-wall. 
  Such 
  an 
  

   operation 
  might 
  conceivably 
  show 
  a 
  negative 
  emergence 
  

   radiation. 
  

  

  It 
  is, 
  in 
  Bact, 
  necessary 
  to 
  remove 
  the 
  ft 
  rays 
  from 
  the 
  

   stream 
  of 
  7 
  ray- 
  by 
  -one- 
  means 
  which 
  is 
  more 
  effective 
  than 
  

   a 
  screen. 
  A 
  powerful 
  magnetic 
  field 
  can 
  considerably 
  purify 
  

   the 
  y-ray 
  stream 
  Bor 
  a 
  short 
  distance 
  along 
  the 
  path. 
  Owing 
  

   to 
  th<- 
  action 
  of 
  the 
  aii'. 
  ft 
  rays 
  will 
  reappear 
  again 
  in 
  

   measurable 
  amount 
  after 
  the 
  rays 
  have 
  traversed 
  a 
  few 
  

   centimetres; 
  nevertheless, 
  we 
  have 
  Found 
  the 
  method 
  to 
  be 
  

   fairly 
  satisfactory 
  so 
  Bar, 
  and 
  will 
  doubtless 
  be 
  able 
  to 
  

   improve 
  it 
  when 
  better 
  information 
  is 
  available 
  for 
  the 
  

   calculation 
  of 
  the 
  remaining 
  errors. 
  The 
  method 
  has 
  already 
  

   been 
  employed 
  by 
  (». 
  rlucera 
  {Bulletin 
  I 
  nter 
  national 
  de 
  

   VAcademie 
  des 
  Sciences 
  de 
  Bohime, 
  1905), 
  but 
  it 
  was 
  

   finally 
  abandoned, 
  because 
  the 
  magnetic 
  lines 
  of 
  induction 
  

   penetrated 
  the 
  ionization-chamber, 
  and 
  affected 
  tbe 
  paths 
  of 
  

   the 
  secondary 
  ft 
  rays, 
  and 
  therefore 
  the 
  magnitude 
  of 
  tbe 
  

   current. 
  We 
  therefore 
  placed 
  a 
  thick 
  iron 
  screen 
  (2*5 
  cm.) 
  

   between 
  the 
  magnet 
  and 
  the 
  chamber 
  ; 
  the 
  y 
  rays 
  passed 
  

   through 
  a 
  hole 
  in 
  the 
  iron. 
  This 
  quite 
  satisfied 
  the 
  purpose 
  

   for 
  which 
  it 
  was 
  intended, 
  and 
  we 
  were 
  therefore 
  provided 
  

   with 
  the 
  means 
  of 
  testing 
  the 
  effects 
  of 
  a 
  7 
  stream 
  fairly 
  free 
  

   from 
  ft 
  rays. 
  We 
  used 
  a 
  magnetic 
  field 
  of 
  about 
  2500 
  units. 
  

   The 
  details 
  and 
  dimensions 
  of 
  the 
  apparatus 
  are 
  shown 
  in 
  

   fig. 
  1. 
  The 
  radium 
  was 
  placed 
  at 
  the 
  bottom 
  of 
  a 
  conical 
  

   hole 
  made 
  in 
  a 
  massive 
  lead 
  block. 
  Plugs 
  of 
  various 
  

   materials 
  and 
  different 
  thicknesses 
  were 
  turned 
  to 
  fit 
  exactly 
  

   in 
  the 
  hole. 
  The 
  screens 
  were 
  brought 
  to 
  the 
  position 
  pp 
  

   by 
  means 
  of 
  a 
  sliding 
  frame, 
  which 
  passed 
  through 
  an 
  

   opening 
  in 
  the 
  side 
  wall 
  of 
  the 
  ionization-chamber. 
  This 
  

   was 
  done 
  to 
  avoid 
  the 
  necessity 
  of 
  opening 
  up 
  the 
  chamber, 
  

   an 
  operation 
  which 
  often 
  makes 
  readings 
  unsteady 
  for 
  a 
  short 
  

   time. 
  The 
  electrode 
  consisted 
  of 
  two 
  concentric 
  circles 
  of 
  

   thick 
  copper 
  wire, 
  connected 
  by 
  short 
  crosspieces, 
  as 
  shown. 
  

   The 
  chamber 
  itself 
  was 
  kept 
  at 
  400 
  volts. 
  The 
  radium 
  was 
  

  

  