﻿H. 
  A. 
  Bumstead 
  — 
  Velocities 
  of 
  Delta 
  Rays. 
  93 
  

  

  slower 
  of 
  these 
  are 
  eliminated 
  and 
  all 
  the 
  remaining 
  ones 
  have 
  

   their 
  velocities 
  reduced 
  ; 
  under 
  these 
  circumstances 
  it 
  appears 
  

   quite 
  impossible 
  to 
  draw 
  any 
  conclusions 
  as 
  to 
  the 
  variation 
  in 
  

   the 
  number 
  of 
  incident 
  S-rays 
  from 
  observations 
  upon 
  the 
  ter- 
  

   tiary 
  electrons. 
  

  

  In 
  attempting 
  to 
  determine 
  the 
  distribution 
  in 
  velocity 
  of 
  

   the 
  swifter 
  S-rays, 
  the 
  essential 
  thing, 
  therefore, 
  is 
  to 
  eliminate 
  

   the 
  effects 
  due 
  to 
  the 
  tertiary 
  electrons. 
  The 
  first 
  method 
  by 
  

   which 
  I 
  attempted 
  to 
  do 
  this 
  was 
  to 
  receive 
  a 
  beam 
  of 
  S-rays 
  

   in 
  a 
  Faraday 
  cylinder, 
  the 
  whole 
  arrangement 
  being 
  in 
  a 
  very 
  

   high 
  vacuum. 
  Two 
  different 
  forms 
  were 
  tried 
  for 
  the 
  source 
  of 
  

   the 
  S-rays 
  ; 
  (A 
  and 
  B, 
  iig. 
  1). 
  In 
  both 
  of 
  these 
  the 
  a-rays 
  from 
  

  

  Fig. 
  1. 
  

  

  1 
  2 
  

  

  CENTIMETERS 
  

  

  the 
  polonium, 
  F, 
  struck 
  the 
  inner 
  walls 
  of 
  a 
  small 
  brass 
  cham- 
  

   ber; 
  a 
  hole 
  in 
  this 
  chamber 
  was 
  placed 
  so 
  as 
  to 
  allow 
  some 
  of 
  

   the 
  S-rays 
  to 
  escape 
  but 
  none 
  of 
  the 
  a-rays. 
  The 
  beam 
  of 
  

   5-rays 
  issuing 
  from 
  this 
  hole 
  was 
  caught 
  in 
  a 
  Faraday 
  cylinder 
  

   after 
  passing 
  through 
  an 
  opposing 
  electric 
  Held 
  ; 
  there 
  were 
  

   suitable 
  diaphragms 
  and 
  earthed 
  screens 
  about 
  the 
  Faraday 
  cyl- 
  

   inder. 
  But 
  although 
  I 
  had 
  in 
  the 
  neighborhood 
  of 
  a 
  millicurie 
  

   of 
  polonium, 
  the 
  arrangement 
  was 
  not 
  sensitive 
  enough 
  to 
  do 
  

   more 
  than 
  indicate 
  the 
  presence 
  of 
  the 
  swifter 
  rays, 
  certainly 
  

   not 
  to 
  measure 
  them. 
  Only 
  a 
  small 
  fraction 
  of 
  the 
  S-rays 
  gen- 
  

   erated 
  in 
  the 
  chamber 
  escaped 
  through 
  the 
  hole, 
  and, 
  even 
  so, 
  

   the 
  beam 
  was 
  so 
  divergent 
  that 
  the 
  Faraday 
  cylinder 
  had 
  to 
  be 
  

   rather 
  large 
  (5*5 
  X 
  4 
  cm 
  ) 
  ; 
  thus 
  its 
  electrostatic 
  capacity 
  was 
  

   considerable 
  and 
  one 
  could 
  not 
  gain 
  any 
  advantage 
  by 
  substi- 
  

   tuting 
  a 
  sensitive 
  electroscope 
  for 
  the 
  quadrant 
  electrometer. 
  

  

  The 
  next 
  attempt 
  was 
  by 
  insulating 
  the 
  source 
  (either 
  A 
  or 
  

   B) 
  from 
  the 
  case 
  and 
  connecting 
  it 
  to 
  the 
  measuring 
  instru- 
  

   ment 
  ; 
  as 
  its 
  capacity 
  was 
  small, 
  an 
  electroscope 
  could 
  now 
  be 
  

   used 
  to 
  advantage. 
  A 
  negative 
  potential 
  applied 
  to 
  the 
  case 
  

   would 
  send 
  back 
  to 
  the 
  source 
  all 
  the 
  electrons 
  whose 
  kinetic 
  

  

  