﻿96 
  H. 
  A. 
  Bumstead 
  — 
  Velocities 
  of 
  Delta 
  Bays. 
  

  

  top, 
  and 
  out 
  of 
  the 
  path 
  of 
  the 
  S-rays. 
  The 
  gauze 
  had 
  5*5 
  

   meshes 
  to 
  the 
  centimeter 
  ; 
  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 
  elec- 
  

   troscope 
  of 
  the 
  Hankel 
  type 
  which 
  has 
  been 
  previously 
  

   described.* 
  The 
  magnitude 
  of 
  the 
  effects 
  obtained, 
  even 
  

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

   electroscope 
  could 
  be 
  used 
  at 
  the 
  very 
  moderate 
  sensitiveness 
  

   of 
  200 
  divisions 
  per 
  volt. 
  Under 
  these 
  circumstances 
  the 
  

   deflections 
  were 
  strictly 
  proportional 
  both 
  to 
  potential 
  and 
  

   current 
  over 
  the 
  whole 
  range 
  of 
  the 
  eye-piece 
  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.f 
  

  

  On 
  account 
  of 
  the 
  emission 
  of 
  8- 
  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 
  

   field 
  this 
  charge 
  is 
  far 
  too 
  great 
  to 
  be 
  measured 
  with 
  the 
  

   arrangements 
  described 
  above. 
  When 
  the 
  case 
  and 
  cage 
  are 
  

   both 
  charged 
  to 
  the 
  same 
  negative 
  potential, 
  all 
  the 
  electrons, 
  

   whose 
  kinetic 
  energy 
  is 
  less 
  than 
  the 
  work 
  which 
  this 
  potential 
  

   can 
  do 
  upon 
  an 
  electron, 
  are 
  returned 
  to 
  the 
  source 
  and 
  only 
  

   the 
  swifter 
  ones 
  escape 
  ; 
  thus 
  the 
  rate 
  at 
  which 
  the 
  source 
  ac- 
  

   quires 
  a 
  positive 
  charge 
  is 
  diminished. 
  When, 
  however, 
  the 
  

   negative 
  potential 
  applied 
  to 
  the 
  gauze 
  and 
  case 
  is 
  more 
  than 
  

   about 
  25 
  volts, 
  the 
  source 
  begins 
  to 
  acquire 
  a 
  negative 
  charge, 
  

   which 
  (as 
  the 
  negative 
  potential 
  is 
  increased) 
  soon 
  reaches 
  a 
  

   maximum 
  and 
  then 
  steadily 
  decreases 
  ; 
  this 
  continues 
  up 
  to, 
  

   and 
  beyond, 
  2000 
  volts. 
  This 
  effect 
  has 
  been 
  shown 
  to 
  be 
  due 
  

   to 
  the 
  emission 
  of 
  tertiary 
  electrons 
  by 
  the 
  case 
  when 
  struck 
  

   by 
  the 
  swifter 
  S-rays 
  ; 
  the 
  tertiary 
  electrons 
  are 
  returned 
  to 
  the 
  

   source 
  of 
  S-rays 
  by 
  the 
  field, 
  and, 
  as 
  they 
  exceed 
  in 
  number 
  

   the 
  S-rays 
  which 
  produce 
  them, 
  the 
  source 
  acquires 
  a 
  resultant 
  

   negative 
  charge, 
  which 
  falls 
  off, 
  however, 
  as 
  more 
  and 
  more 
  

   8-electrons 
  are 
  restrained 
  from 
  reaching 
  the 
  case.J 
  

  

  * 
  Bumstead, 
  this 
  Journal, 
  xxxii, 
  403, 
  1911 
  ; 
  Phil. 
  Mag., 
  xxii, 
  910, 
  1911. 
  

  

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

   protect 
  any 
  sensitive 
  electroscope 
  against 
  sudden 
  changes 
  of 
  tempera- 
  

   ture. 
  With 
  the 
  electroscope 
  mentioned, 
  satisfactory 
  protection 
  is 
  given 
  by 
  

   covering 
  it 
  with 
  felt 
  about 
  )4 
  inch 
  thick, 
  and 
  by 
  setting 
  it 
  up 
  in 
  a 
  wooden 
  

   box 
  (with 
  an 
  open 
  front), 
  to 
  keep 
  off 
  drafts 
  in 
  some 
  degree. 
  A 
  glass 
  win- 
  

   dow 
  in 
  the 
  back 
  of 
  the 
  box 
  admits 
  light 
  to 
  the 
  gold-leaf. 
  

  

  % 
  Bumstead 
  and 
  McGougan, 
  1. 
  c, 
  §3. 
  

  

  