﻿H. 
  A. 
  Bumstead 
  — 
  Velocities 
  of 
  Delta 
  Bays. 
  107 
  

  

  Electronic 
  current 
  at 
  —40 
  volts.. 
  43 
  

  

  " 
  " 
  " 
  — 
  20 
  " 
  100 
  

  

  " 
  " 
  " 
  — 
  2 
  " 
  1700 
  

  

  " 
  " 
  " 
  " 
  2700 
  

  

  " 
  " 
  " 
  -f 
  40 
  " 
  . 
  ... 
  5800 
  

  

  Current 
  carried 
  by 
  a-rays 
  - 
  _ 
  200 
  

  

  Minimum 
  ionic 
  current 
  . 
  _ 
  10 
  

  

  It 
  will 
  be 
  seen 
  that 
  the 
  swifter 
  electrons 
  with 
  which 
  we 
  

   have 
  been 
  principally 
  concerned 
  form 
  only 
  a 
  small 
  part 
  of 
  the 
  

   total 
  number 
  which 
  leave 
  a 
  metal 
  when 
  it 
  is 
  struck 
  by 
  a-rays. 
  

   But 
  the 
  fact 
  that 
  a-rays 
  can 
  cause 
  electrons 
  to 
  be 
  projected 
  

   with 
  such 
  speeds 
  is 
  undoubtedly 
  a 
  fact 
  of 
  considerable 
  import- 
  

   ance, 
  whether 
  these 
  electrons 
  be 
  few 
  or 
  many 
  ; 
  it 
  must 
  have 
  

   some 
  bearing 
  upon 
  the 
  theory 
  of 
  ionization 
  by 
  a-par 
  tides 
  and 
  

   of 
  their 
  passage 
  through 
  matter. 
  It 
  is 
  not 
  altogether 
  surpris- 
  

   ing 
  that 
  a-particles 
  should 
  cause 
  electrons 
  to 
  be 
  projected 
  with 
  

   velocities 
  corresponding 
  to 
  some 
  hundreds 
  of 
  volts. 
  Accord- 
  

   ing 
  to 
  the 
  theory 
  of 
  Einstein, 
  the 
  energy 
  of 
  electrons 
  projected 
  

   under 
  the 
  influence 
  of 
  ultra-violet 
  light 
  is 
  a 
  linear 
  function 
  of 
  

   the 
  frequency 
  of 
  the 
  light. 
  This 
  theory 
  has 
  been 
  extended 
  

   with 
  some 
  degree 
  of 
  success 
  to 
  the 
  electronic 
  emission 
  caused 
  

   by 
  Rontgen 
  rays, 
  taking, 
  instead 
  of 
  the 
  frequency, 
  the 
  time 
  

   occupied 
  by 
  the 
  pulse 
  in 
  passing 
  over 
  an 
  electron, 
  and 
  esti- 
  

   mating 
  this 
  as 
  well 
  as 
  can 
  be 
  done 
  in 
  the 
  present 
  state 
  of 
  

   knowledge. 
  If 
  we 
  assume 
  that 
  the 
  effective 
  field 
  about 
  an 
  

   a-particle 
  has 
  the 
  diameter 
  of 
  an 
  atom, 
  10" 
  8cm 
  , 
  then, 
  since 
  its 
  

  

  cm 
  

   velocity 
  is 
  about 
  2 
  X 
  10 
  9 
  — 
  , 
  its 
  time 
  of 
  passage 
  over 
  an 
  dec- 
  

  

  sec 
  

  

  tron 
  will 
  be 
  i 
  X 
  10" 
  17 
  seconds. 
  The 
  frequency 
  of 
  ultra-violet 
  

  

  light 
  is 
  about 
  | 
  X 
  10 
  ~ 
  15 
  seconds, 
  so 
  that 
  we 
  might 
  expect 
  the 
  

  

  maximum 
  energy 
  of 
  the 
  S-rays 
  to 
  be 
  of 
  the 
  order 
  of 
  100 
  times 
  

  

  that 
  of 
  photoelectric 
  electrons. 
  

  

  It 
  has 
  been 
  shown 
  that 
  the 
  number 
  of 
  slow 
  S-electrons 
  

  

  varies 
  with 
  the 
  speed 
  of 
  the 
  a-rays 
  in 
  much 
  the 
  same 
  manner 
  

  

  as 
  the 
  number 
  of 
  ions 
  produced 
  in 
  a 
  gas. 
  I 
  have 
  not 
  yet 
  been 
  

  

  able 
  to 
  determine 
  whether 
  this 
  is 
  so 
  with 
  the 
  swifter 
  S-rays, 
  

  

  nor 
  how 
  their 
  distribution 
  in 
  velocity 
  varies 
  (if 
  at 
  all) 
  with 
  the 
  

  

  speed 
  of 
  the 
  a-rays. 
  Knowledge 
  of 
  this 
  sort 
  might 
  throw 
  

  

  considerable 
  light 
  upon 
  the 
  relations 
  between 
  the 
  electrons 
  of 
  

  

  various 
  speeds, 
  and 
  upon 
  the 
  mechanism 
  of 
  ionization 
  by 
  

  

  a-rays, 
  about 
  which 
  very 
  little 
  is 
  known 
  at 
  present. 
  

  

  Summary. 
  

  

  1. 
  When 
  a-rays 
  fall 
  upon 
  a 
  metal, 
  electrons 
  are 
  emitted 
  with 
  

  

  velocities 
  varying 
  continuously 
  from 
  a 
  very 
  small 
  value 
  to 
  

  

  cm 
  

   more 
  than 
  2*7 
  XlO 
  9 
  — 
  , 
  which 
  corresponds 
  to 
  a 
  potential 
  differ- 
  

   sec 
  

  

  Am. 
  Jour. 
  Sci.— 
  Fourth 
  Series, 
  Vol. 
  XXXV, 
  No. 
  212.— 
  August, 
  1913. 
  

  

  