﻿592 
  Mr. 
  A. 
  B. 
  Wood 
  on 
  the 
  Range 
  of 
  the 
  

  

  hemispheres, 
  the 
  inner 
  one 
  of 
  fine-mesh 
  copper 
  gauze, 
  insu- 
  

   lated 
  from 
  each 
  other 
  by 
  means 
  of 
  three 
  sulphur 
  beads. 
  The 
  

   upper 
  brass 
  hemisphere 
  is 
  insulated 
  from 
  and 
  connected 
  

   through 
  the 
  glass 
  plate 
  to 
  the 
  leaf 
  of 
  a 
  sensitive 
  electroscope 
  

   which 
  is 
  made 
  perfectly 
  airtight. 
  As. 
  before, 
  the 
  active 
  

   deposit 
  was 
  obtained 
  on 
  a 
  small 
  polished 
  platinum 
  plate 
  

   (about 
  3 
  mm. 
  diam.) 
  placed 
  at 
  the 
  centre 
  of 
  the 
  hemispheres. 
  

   This 
  apparatus 
  was 
  so 
  arranged 
  that 
  the 
  electroscope 
  case, 
  

   brass 
  hemisphere, 
  gauze 
  hemisphere, 
  and 
  platinum 
  plate 
  

   could 
  be 
  raised 
  to 
  different 
  potentials 
  as 
  required. 
  In 
  the 
  

   earlier 
  experiments 
  a 
  P.D. 
  of 
  240 
  volts 
  across 
  an 
  ionization 
  

   chamber 
  of 
  depth 
  1 
  mm. 
  was 
  employed. 
  At 
  low 
  pressures 
  

   the 
  production 
  of 
  ions 
  by 
  collision, 
  consequent 
  on 
  this 
  strong 
  

   field, 
  made 
  the 
  interpretation 
  of 
  results 
  rather 
  complex, 
  

   hence 
  it 
  became 
  necessary 
  to 
  work 
  with 
  a 
  small 
  P.D., 
  about 
  

   40 
  volts, 
  between 
  the 
  hemispheres 
  about 
  3 
  mm. 
  apart. 
  In 
  

   the 
  final 
  arrangement 
  the 
  case 
  of 
  the 
  electroscope 
  was 
  con- 
  

   nected 
  to 
  the 
  negative 
  electrode 
  of 
  a 
  battery 
  of 
  240 
  volts, 
  

   the 
  brass 
  hemisphere 
  and 
  leaf 
  being 
  earthed. 
  The 
  induced 
  

   charge 
  of 
  240 
  volts 
  on 
  the 
  leaf 
  caused 
  it 
  to 
  diverge 
  at 
  a 
  

   sensitive 
  angle. 
  The 
  gauze 
  hemisphere 
  was 
  connected 
  to 
  

   the 
  negative 
  electrode 
  of 
  a 
  battery 
  of 
  40 
  volts 
  and 
  the 
  active 
  

   plate 
  a 
  was 
  earthed. 
  This 
  arrangement 
  proved 
  quite 
  satis- 
  

   factory, 
  and 
  ionization 
  by 
  collision 
  was 
  reduced 
  to 
  a 
  minimum. 
  

   In 
  connexion 
  with 
  this 
  apparatus 
  two 
  taps 
  were 
  arranged 
  

   so 
  that 
  dry 
  gas, 
  air 
  or 
  hydrogen, 
  could 
  be 
  admitted 
  in 
  very 
  

   small 
  amounts. 
  Thus 
  it 
  was 
  possible 
  to 
  increase 
  the 
  pressure 
  

   from 
  a 
  minimum 
  value 
  by 
  small 
  steps 
  to 
  any 
  desired 
  pressure 
  

   — 
  the 
  ionization 
  being 
  measured 
  over 
  a 
  considerable 
  range 
  

   of 
  pressures 
  and 
  a 
  curve 
  connecting 
  pressure 
  and 
  ionization 
  

   obtained. 
  In 
  all 
  experiments 
  the 
  upper 
  chamber 
  containing 
  

   the 
  leaf 
  of 
  the 
  electroscope 
  was 
  evacuated 
  to 
  reduce 
  to 
  a 
  

   minimum 
  the 
  ionization 
  produced 
  by 
  y 
  rays 
  penetrating 
  the 
  

   glass 
  plate. 
  

  

  (a) 
  Recoil 
  from 
  Thorium 
  Active 
  Deposit 
  in 
  Air. 
  

   Fig. 
  4, 
  curve 
  A, 
  shows 
  the 
  result 
  of 
  an 
  experiment 
  in 
  air. 
  

   A 
  distinct 
  " 
  hump 
  " 
  is 
  observed 
  at 
  low 
  pressures 
  ; 
  this 
  

   gradually 
  merges 
  into 
  a 
  straight 
  line 
  which, 
  when 
  produced, 
  

   passes 
  through 
  the 
  origin. 
  The 
  curve 
  shown 
  in 
  the 
  figure 
  

   bears 
  a 
  strong 
  resemblance 
  to 
  those 
  obtained 
  by 
  Werten 
  stein 
  

   for 
  the 
  recoil 
  of 
  radium 
  B, 
  but 
  in 
  this 
  case 
  the 
  hump 
  is 
  

   more 
  pronounced. 
  It 
  will 
  be 
  noticed 
  that 
  the 
  maximum 
  of 
  

   the 
  hump 
  occurs 
  at 
  a 
  pressure 
  of 
  about 
  2 
  mm,, 
  and 
  the 
  linear 
  

   portion 
  of 
  the 
  curve 
  begins 
  at 
  a 
  pressure 
  of 
  about 
  5 
  mm. 
  

   These 
  pressures 
  for 
  air 
  were 
  practically 
  the 
  same 
  in 
  all 
  

  

  