﻿696 
  Quality 
  of 
  the 
  Secondary 
  Ionization 
  due 
  to 
  /3 
  Rays. 
  

  

  may, 
  therefore, 
  take 
  the 
  figures 
  in 
  any 
  vertical 
  column 
  of 
  

   the 
  first 
  table 
  given 
  above 
  as 
  showing 
  the 
  effects 
  of 
  the 
  

   secondary 
  radiation 
  at 
  various 
  distances 
  from 
  the 
  radiator. 
  

   If- 
  we 
  plot 
  an 
  ionization-distance 
  curve 
  the 
  whole 
  area 
  between 
  

   the 
  curve 
  and 
  the 
  axes 
  is 
  the 
  true 
  measure 
  of 
  the 
  ionization 
  

   due 
  to 
  the 
  returned 
  rays. 
  The 
  usual 
  plan 
  is 
  to 
  accept 
  as 
  a 
  

   measure 
  the 
  one 
  figure 
  at 
  the 
  head 
  of 
  the 
  column. 
  

  

  We 
  find 
  that 
  in 
  each 
  case 
  the 
  curve 
  may 
  be 
  nearly 
  expressed 
  

   as 
  the 
  sum 
  of 
  two 
  exponentials 
  as 
  Ae~ 
  klX 
  + 
  ~Be~ 
  Xo 
  - 
  a 
  '. 
  Whether 
  

   this 
  has 
  a 
  physical 
  meaning 
  or 
  not 
  does 
  not 
  concern 
  our 
  

   immediate 
  argument. 
  Commencing 
  with 
  one 
  thickness 
  of 
  

   tinfoil 
  on 
  the 
  dome, 
  the 
  actual 
  figures 
  are 
  : 
  — 
  

  

  I. 
  

  

  II. 
  

  

  III. 
  

  

  IV. 
  

  

  Pb 
  

  

  — 
  6&r 
  -13-6.1 
  

  

  564* 
  + 
  560* 
  

  

  200 
  

  

  200 
  

  

  Ag 
  

  

  -57x 
  -Ux 
  

  

  492* 
  + 
  337* 
  

  

  125 
  

  

  140 
  

  

  Zn 
  

  

  -57x 
  — 
  Ux 
  

  

  409* 
  +218* 
  

  

  83 
  

  

  98'5 
  

  

  Te 
  

  

  -54c 
  -Ux 
  

  

  365* 
  +154* 
  

  

  61-5 
  

  

  76*5 
  

  

  s 
  

  

  — 
  59x 
  - 
  13-oz 
  

  

  264* 
  + 
  78* 
  

  

  35 
  

  

  43 
  

  

  Al 
  

  

  — 
  73x 
  — 
  Ux 
  

  

  218* 
  + 
  58* 
  

  

  19-5 
  

  

  27-5 
  

  

  C 
  

  

  -73x 
  — 
  14r 
  

  

  117* 
  + 
  23* 
  

  

  

  

  

  

  The 
  area 
  of 
  the 
  curve 
  is 
  given 
  by 
  A/Xj 
  + 
  B/X 
  2 
  , 
  and 
  the 
  

   relative 
  values 
  of 
  the 
  areas 
  are 
  placed 
  in 
  the 
  third 
  column, 
  

   that 
  of 
  C 
  being 
  first 
  subtracted 
  and 
  that 
  of 
  Pb 
  being 
  set 
  

   equal 
  to 
  200. 
  In 
  the 
  last 
  column 
  are 
  the 
  corresponding 
  

   figures 
  when 
  the 
  amount 
  of 
  radiation 
  is 
  measured 
  by 
  A 
  + 
  B, 
  

   i. 
  e., 
  the 
  ionization 
  found 
  when 
  one 
  layer 
  of 
  tinfoil 
  is 
  placed 
  

   over 
  the 
  chamber. 
  It 
  is 
  clear 
  that 
  there 
  is 
  a 
  considerable 
  

   difference. 
  

  

  When 
  tinfoil 
  is 
  placed 
  over 
  the 
  radium 
  itself 
  so 
  as 
  to 
  

   cut 
  out 
  a 
  large 
  proportion 
  of 
  the 
  slower 
  (3 
  rays, 
  the 
  secondary 
  

   radiation 
  is 
  also 
  hardened 
  correspondingly. 
  

  

  The 
  figures 
  here 
  given 
  are 
  not 
  sufficiently 
  accurate 
  to 
  be 
  

   taken 
  as 
  absolute. 
  The 
  measurements 
  were 
  made 
  as 
  carefully 
  

  

  