﻿94 
  H. 
  A. 
  Bumstead 
  — 
  Velocities 
  of 
  Delta 
  Bays. 
  

  

  energy 
  was 
  not 
  sufficient 
  to 
  overcome 
  the 
  opposing 
  potential 
  

   difference. 
  Here 
  the 
  difficulty 
  was 
  to 
  prevent 
  the 
  tertiary 
  

   electrons, 
  set 
  up 
  by 
  the 
  impact 
  of 
  the 
  8-rays 
  upon 
  the 
  case, 
  

   from 
  being 
  carried 
  to 
  the 
  source 
  by 
  the 
  electric 
  Held. 
  I 
  first 
  

   tried 
  to 
  obviate 
  this 
  by 
  means 
  of 
  a 
  magnetic 
  field. 
  A 
  long 
  

   cylindrical 
  case 
  was 
  used 
  (the 
  one 
  shown 
  in 
  fig. 
  2), 
  with 
  the 
  

   source 
  of 
  S-rays 
  near 
  the 
  top 
  and 
  the 
  rays 
  striking 
  the 
  bottom 
  

   and 
  the 
  lower 
  part 
  of 
  the 
  sides 
  of 
  the 
  case. 
  A 
  magnetic 
  field 
  

   of 
  about 
  50 
  gauses 
  was 
  applied 
  to 
  this 
  part 
  of 
  the 
  case. 
  To 
  

   prevent 
  this 
  field 
  as 
  much 
  as 
  possible 
  from 
  having 
  an 
  effect 
  

   upon 
  the 
  emission 
  of 
  the 
  S-rays 
  themselves, 
  the 
  source 
  was 
  

   inclosed 
  in 
  a 
  sort 
  of 
  upper 
  chamber 
  made 
  of 
  a 
  plate 
  and 
  a 
  ring 
  

   of 
  soft 
  Norway 
  iron, 
  l'5 
  cm 
  thick 
  ; 
  (this 
  is 
  not 
  shown 
  in 
  fig. 
  2). 
  

   A 
  tapered 
  hole 
  in 
  the 
  plate 
  allowed 
  the 
  beam 
  of 
  S-rays 
  to 
  pass 
  

   into 
  the 
  lower 
  chamber 
  ; 
  both 
  chambers 
  were 
  coated 
  with 
  soot. 
  

   Preliminary 
  measurements 
  of 
  the 
  magnetic 
  field 
  in 
  the 
  upper 
  

   chamber 
  by 
  means 
  of 
  a 
  swinging 
  needle 
  showed 
  that, 
  with 
  a 
  

   field 
  of 
  50 
  gausses 
  near 
  the 
  bottom 
  of 
  the 
  case, 
  the 
  field 
  near 
  

   the 
  source 
  was 
  not 
  more 
  than 
  0*5 
  gauss. 
  

  

  This 
  method 
  was 
  not 
  wholly 
  unsuccessful. 
  In 
  a 
  charcoal 
  

   vacuum 
  which 
  had 
  been 
  maintained 
  for 
  several 
  days, 
  the 
  source 
  

   lost 
  negative 
  electricity 
  in 
  the 
  face 
  of 
  an 
  opposing 
  potential 
  of 
  

   300 
  volts, 
  but 
  beyond 
  that 
  it 
  acquired 
  a 
  negative 
  charge. 
  The 
  

   form 
  of 
  the 
  relation 
  between 
  current 
  and 
  potential 
  for 
  the 
  

   lower 
  voltages 
  was 
  quite 
  similar 
  to 
  those 
  obtained 
  later 
  with 
  

   the 
  final 
  form 
  of 
  apparatus 
  ; 
  with 
  potentials 
  over 
  250 
  volts, 
  

   however, 
  the 
  effects 
  of 
  the 
  tertiary 
  electrons 
  began 
  to 
  be 
  con- 
  

   spicuous. 
  Moreover, 
  on 
  account 
  of 
  the 
  limited 
  beam 
  of 
  S-rays 
  

   the 
  values 
  of 
  the 
  currents 
  were 
  small 
  and 
  necessitated 
  the 
  use 
  

   of 
  a 
  very 
  sensitive, 
  and 
  consequently 
  troublesome, 
  electro- 
  

   scope. 
  

  

  §3- 
  

   The 
  final 
  form 
  of 
  the 
  apparatus 
  is 
  shown 
  in 
  fig. 
  2. 
  The 
  

   source 
  of 
  8-rays 
  is 
  the 
  shallow, 
  open, 
  cylindrical 
  box, 
  B, 
  made 
  

   of 
  brass 
  ; 
  in 
  some 
  of 
  the 
  later 
  experiments, 
  in 
  which 
  a 
  mag- 
  

   netic 
  field 
  was 
  used, 
  the 
  box 
  was 
  surrounded 
  by 
  a 
  fiat 
  ring 
  of 
  

   brass, 
  as 
  shown 
  in 
  the 
  figure, 
  giving 
  to 
  the 
  source 
  the 
  shape 
  of 
  

   a 
  sailor's 
  hat. 
  Below, 
  and 
  opposite 
  the 
  middle 
  of 
  this 
  box, 
  a 
  

   thin 
  brass 
  arm 
  supported 
  a 
  cylindrical 
  copper 
  plug, 
  P, 
  with 
  a 
  

   deposit 
  of 
  polonium 
  on 
  its 
  upper 
  surface, 
  which 
  was 
  4 
  mm 
  

   in 
  diameter.* 
  The 
  end 
  of 
  this 
  plug 
  projected 
  into 
  the 
  box 
  

   so 
  that 
  none 
  of 
  the 
  a-rays 
  escaped, 
  but 
  fell 
  upon 
  the 
  top 
  and 
  

  

  *I 
  am 
  again 
  indebted 
  to 
  Professor 
  Boltwood 
  for 
  the 
  polonium 
  used 
  in 
  

   these 
  experiments. 
  For 
  three 
  successive 
  years 
  he 
  has 
  most 
  kindly 
  supplied 
  

   me 
  with 
  the 
  annual 
  crop 
  of 
  polonium 
  grown 
  from 
  a 
  preparation 
  of 
  radio-lead 
  

   in 
  his 
  possession. 
  

  

  