﻿92 
  H. 
  A. 
  Bumstead 
  — 
  Velocities 
  of 
  Delta 
  Hays. 
  

  

  Last 
  year 
  it 
  was 
  shown 
  by 
  Dr. 
  McGougan 
  and 
  the 
  present 
  

   writer* 
  that 
  electrons 
  were 
  present 
  in 
  a 
  beam 
  of 
  S-rays 
  which 
  

   had 
  much 
  greater 
  velocities 
  than 
  any 
  of 
  the 
  above 
  estimates 
  ; 
  

   an 
  opposing 
  potential 
  difference 
  of 
  1700 
  volts 
  was 
  not 
  sufficient 
  

   to 
  stop 
  all 
  of 
  them, 
  and 
  very 
  marked 
  effects 
  were 
  produced 
  by 
  

   electrons 
  having 
  velocities 
  corresponding 
  to 
  several 
  hundred 
  

   volts. 
  The 
  present 
  paper 
  contains 
  the 
  results 
  of 
  some 
  further 
  

   experiments 
  upon 
  these 
  swifter 
  rays. 
  It 
  will 
  be 
  seen 
  that 
  there 
  

   is 
  no 
  gap 
  between 
  the 
  swifter 
  and 
  the 
  slower 
  electrons, 
  but 
  that 
  

   all 
  intermediate 
  speeds 
  are 
  found 
  between 
  the 
  highest 
  and 
  the 
  

   lowest. 
  It 
  seems 
  reasonable, 
  therefore, 
  to 
  include 
  under 
  the 
  

   name 
  u 
  delta 
  rays 
  " 
  all 
  the 
  electrons 
  which 
  are 
  projected 
  from 
  

   the 
  atoms 
  of 
  bodies 
  by 
  the 
  direct 
  action 
  of 
  the 
  a-rays, 
  the 
  

   recently 
  discovered 
  swift 
  ones 
  as 
  well 
  as 
  the 
  slower 
  ones 
  previ- 
  

   ously 
  known 
  ; 
  this 
  nomenclature 
  will 
  be 
  adopted 
  in 
  the 
  present 
  

   paper. 
  It 
  is, 
  however, 
  to 
  be 
  observed 
  that 
  some, 
  at 
  least, 
  of 
  the 
  

   slower 
  electrons 
  (under 
  10 
  volts) 
  must 
  be 
  caused, 
  not 
  directly 
  

   by 
  the 
  a-rays, 
  but 
  by 
  the 
  swifter 
  S-rays. 
  When 
  there 
  is 
  occa- 
  

   sion 
  to 
  refer 
  to 
  these 
  they 
  will 
  be 
  called 
  tertiary 
  rays. 
  It 
  is 
  

   impossible 
  at 
  present 
  to 
  make 
  a 
  sharp 
  distinction 
  between 
  the 
  

   slower 
  8-rays 
  and 
  the 
  tertiary 
  rays 
  which 
  come 
  from 
  the 
  source 
  

   of 
  S-rays, 
  or 
  to 
  determine 
  the 
  proportion 
  of 
  each 
  ; 
  so 
  that 
  

   in 
  all 
  numerical 
  estimates 
  the 
  tertiary 
  rays 
  must 
  be 
  included 
  

   among 
  the 
  S-rays. 
  

  

  §2. 
  

  

  Tertiary 
  electrons 
  are 
  emitted 
  also 
  by 
  any 
  body 
  on 
  which 
  

   S-rays 
  fall 
  and 
  in 
  numbers 
  considerably 
  in 
  excess 
  of 
  the 
  swifter 
  

   S-electrons 
  which 
  cause 
  them. 
  When 
  an 
  electric 
  field 
  is 
  used 
  

   to 
  hold 
  back 
  the 
  slower 
  8-electrons, 
  this 
  is 
  the 
  most 
  conspicuous 
  

   effect 
  of 
  the 
  remaining 
  swift 
  rays 
  ; 
  for 
  the 
  field 
  which 
  opposes 
  

   the 
  S-rays 
  assists 
  the 
  tertiary 
  electrons 
  to 
  escape, 
  and 
  their 
  

   larger 
  number 
  magnifies 
  the 
  experimental 
  effect. 
  The 
  unsus- 
  

   pected 
  presence 
  of 
  this 
  phenomenon 
  has 
  undoubtedly 
  had 
  an 
  

   influence 
  upon 
  the 
  results 
  obtained 
  in 
  previous 
  experiments 
  

   upon 
  6-rays, 
  and 
  helps 
  to 
  explain 
  the 
  discrepancies 
  which 
  have 
  

   appeared 
  in 
  the 
  work 
  of 
  different 
  investigators. 
  

  

  Unfortunately 
  this 
  effect, 
  which 
  is 
  easy 
  to 
  observe 
  and 
  meas- 
  

   ure, 
  does 
  not 
  lend 
  itself 
  readily 
  to 
  a 
  quantitative 
  study 
  of 
  the 
  

   swifter 
  S-rays. 
  The 
  number 
  of 
  secondary 
  electrons, 
  due 
  to 
  a 
  

   single 
  incident 
  electron, 
  varies 
  markedly, 
  and 
  not 
  in 
  a 
  very 
  

   simple 
  way, 
  with 
  the 
  speed 
  of 
  the 
  latter, 
  f 
  In 
  a 
  beam 
  of 
  S-rays 
  

   one 
  has 
  a 
  complex 
  of 
  electrons 
  of 
  many 
  different 
  speeds, 
  and 
  

   when 
  the 
  opposing 
  electric 
  field 
  is, 
  for 
  example, 
  increased, 
  the 
  

  

  * 
  Bumstead 
  and 
  McGougan. 
  this 
  Journal, 
  xxxiv, 
  309, 
  1912; 
  Phil. 
  Mag., 
  

   xxiv, 
  474, 
  1912. 
  

  

  fGehrts, 
  Ann. 
  d. 
  Phys., 
  xxxvi, 
  1000 
  et 
  seq., 
  1911. 
  

  

  