﻿238 
  Prof. 
  H. 
  A. 
  Bumstead 
  on 
  the 
  

  

  the 
  latter 
  and 
  the 
  smallnesa 
  of 
  the 
  obstacle 
  presented 
  by 
  the 
  

   polonium 
  and 
  its 
  support 
  enabled 
  a 
  large 
  proportion 
  of 
  

   the 
  S-rays 
  and 
  tertiary 
  rays 
  to 
  get 
  away. 
  This 
  source 
  was 
  

   enclosed 
  in 
  a 
  large 
  cylindrical 
  brass 
  case 
  which 
  could 
  be 
  

   highly 
  exhausted 
  by 
  the 
  help 
  of 
  charcoal 
  and 
  liquid 
  air 
  ; 
  the 
  

   charcoal 
  bulb 
  was 
  placed 
  between 
  the 
  evacuated 
  chamber 
  and 
  

   the 
  pump 
  and 
  gauge, 
  and 
  was 
  so 
  constructed 
  as 
  to 
  free 
  the 
  

   chamber 
  from 
  mercury 
  vapour 
  by 
  its 
  distillation 
  into 
  the 
  

   cold 
  bulb. 
  Within 
  the 
  case 
  and 
  | 
  cm. 
  from 
  its 
  walls 
  was 
  

   a 
  cylindrical 
  cage 
  of 
  bronze 
  wire-gauze 
  which 
  was 
  insulated 
  

   from 
  the 
  case 
  and 
  could 
  be 
  separately 
  charged 
  by 
  means 
  of 
  

   an 
  external 
  electrode. 
  It 
  was 
  found 
  to 
  be 
  important 
  that 
  the 
  

   insulators 
  supporting 
  the 
  cage 
  should 
  not 
  be 
  struck 
  by 
  the 
  

   S-rays; 
  if 
  they 
  were, 
  they 
  acquired 
  charges 
  which 
  gave 
  rise 
  

   to 
  erratic 
  results. 
  Accordingly 
  the 
  cage 
  was 
  supported 
  by 
  

   three 
  small 
  pieces 
  of 
  ebonite 
  near 
  the 
  top, 
  and 
  out 
  of 
  the 
  path 
  

   of 
  the 
  S-rays. 
  The 
  gauze 
  had 
  5*5 
  meshes 
  to 
  the 
  centimetre 
  ; 
  

   of 
  its 
  entire 
  area 
  83 
  per 
  cent, 
  was 
  open, 
  the 
  remaining 
  

   17 
  per 
  cent, 
  was 
  occupied 
  by 
  the 
  wires. 
  

  

  The 
  rod 
  which 
  supported 
  the 
  source, 
  B, 
  was 
  insulated 
  from 
  

   the 
  case 
  and 
  provided 
  with 
  an 
  earthed 
  guard-tube 
  in 
  the 
  

   usual 
  manner. 
  It 
  was 
  connected 
  to 
  the 
  gold-leaf 
  of 
  a 
  

   sensitive 
  electroscope 
  of 
  the 
  Hankel 
  type 
  which 
  has 
  been 
  pre- 
  

   viously 
  described 
  *» 
  The 
  magnitude 
  of 
  the 
  effects 
  obtained, 
  

   even 
  when 
  all 
  the 
  slower 
  8-rays 
  were 
  stopped, 
  was 
  such 
  that 
  

   the 
  electroscope 
  could 
  be 
  used 
  at 
  the 
  very 
  moderate 
  sensi- 
  

   tiveness 
  of 
  200 
  divisions 
  per 
  volt. 
  Under 
  these 
  circumstances 
  

   the 
  deflexions 
  were 
  strictly 
  proportional 
  both 
  to 
  potential 
  and 
  

   current 
  over 
  the 
  whole 
  range 
  of 
  the 
  eyepiece 
  scale 
  in 
  the 
  

   microscope 
  (100 
  divisions); 
  the 
  sensitiveness 
  remained 
  quite 
  

   constant, 
  so 
  that 
  there 
  was 
  no 
  need 
  of 
  continual 
  checking 
  up 
  

   by 
  means 
  of 
  a 
  potentiometer. 
  When 
  changes 
  in 
  the 
  zero 
  

   position 
  occurred, 
  they 
  were 
  slow 
  and 
  steady 
  and 
  could 
  easily 
  

   be 
  taken 
  into 
  account, 
  and 
  they 
  caused 
  no 
  alteration 
  in 
  the 
  

   sensitiveness 
  ; 
  often 
  the 
  zero 
  would 
  not 
  vary 
  more 
  than 
  one 
  

   or 
  two 
  divisions 
  in 
  an 
  entire 
  half-day's 
  work 
  t* 
  

  

  On 
  account 
  of 
  the 
  emission 
  of 
  o 
  and 
  tertiary 
  rays, 
  the 
  

   source 
  as 
  soon 
  as 
  it 
  is 
  insulated 
  acquires 
  a 
  positive 
  charge 
  

   which 
  increases 
  with 
  the 
  time. 
  When 
  there 
  is 
  no 
  opposing 
  

  

  * 
  Bumstead, 
  Phil. 
  Mag. 
  xxii. 
  p. 
  910 
  (1911) 
  : 
  Am. 
  Journ. 
  Sci. 
  xxxii. 
  

   p. 
  403 
  (1911), 
  

  

  + 
  In 
  order 
  to 
  secure 
  fair 
  steadiness 
  of 
  the 
  zero 
  reading, 
  it 
  is 
  necessary 
  

   to 
  protect 
  any 
  sensitive 
  electroscope 
  against 
  sudden 
  changes 
  of 
  tempe- 
  

   rature. 
  With 
  the 
  electroscope 
  mentioned 
  satisfactory 
  protection 
  is 
  given 
  

   by 
  covering 
  it 
  with 
  felt 
  about 
  \ 
  inch 
  thick, 
  and 
  by 
  setting 
  it 
  up 
  in 
  a 
  

   wooden 
  box 
  (with 
  an 
  open 
  front), 
  to 
  keep 
  off 
  draughts 
  in 
  some 
  degree. 
  

   A 
  glass 
  window 
  in 
  the 
  back 
  of 
  the 
  box 
  admits 
  light 
  to 
  the 
  gold-leaf. 
  

  

  