﻿Wellisch 
  and 
  Woodrow 
  — 
  Columnar' 
  Ionization. 
  217 
  

  

  cular 
  brass 
  plate 
  A, 
  which 
  was 
  connected 
  to 
  one 
  pair 
  of 
  quad- 
  

   rants 
  of 
  a 
  Dolezalek 
  electrometer. 
  The 
  lower 
  gauze 
  G 
  was 
  

   inserted 
  as 
  usual, 
  to 
  avoid 
  disturbances 
  due 
  to 
  diffusion 
  of 
  ions 
  

   into 
  the 
  region 
  AB. 
  

  

  For 
  the 
  horizontal 
  transverse 
  field 
  two 
  brass 
  plates 
  were 
  

   employed, 
  one 
  C 
  (8*5 
  X 
  2 
  - 
  5 
  cm 
  ) 
  counected 
  to 
  the 
  battery, 
  and 
  

   the 
  other 
  D 
  (7*2 
  X 
  0'4 
  cm 
  ), 
  which 
  connected 
  with 
  the 
  electrom- 
  

   eter. 
  The 
  latter 
  electrode 
  was 
  surrounded 
  by 
  an 
  earthed 
  plate 
  

   E, 
  which 
  served 
  as 
  a 
  guard 
  ring. 
  

  

  Both 
  the 
  longitudinal 
  and 
  the 
  transverse 
  fields 
  were 
  so 
  con- 
  

   structed 
  that 
  they 
  could 
  be 
  inserted 
  in 
  the 
  same 
  containing 
  

   vessel, 
  as 
  shown 
  in 
  the 
  diagram. 
  

  

  A 
  thin 
  him 
  of 
  polonium 
  deposited 
  on 
  a 
  copper 
  plug, 
  4 
  mm 
  in 
  

   diameter, 
  which 
  had 
  been 
  prepared 
  previously 
  by 
  Prof. 
  Bolt- 
  

   wood, 
  was 
  employed 
  as 
  the 
  alpha-ray 
  source. 
  This 
  plug 
  was 
  

   placed 
  on 
  the 
  carrier 
  R, 
  which 
  could 
  be 
  moved 
  vertically 
  by 
  a 
  

   screw, 
  S. 
  A 
  scale 
  and 
  divided 
  head 
  enabled 
  one 
  to 
  make 
  a 
  

   very 
  accurate 
  determination 
  of 
  the 
  distance 
  of 
  the 
  polonium 
  

   from 
  the 
  center 
  of 
  the 
  ionization 
  vessel. 
  A 
  narrow 
  beam 
  of 
  

   alpha-rays 
  was 
  always 
  employed 
  ; 
  this 
  was 
  obtained 
  by 
  placing 
  

   over 
  the 
  polonium 
  a 
  tine 
  slit 
  or 
  a 
  series 
  of 
  fine 
  ' 
  canals' 
  as 
  de- 
  

   scribed 
  below. 
  

  

  The 
  electrometer 
  was 
  of 
  the 
  Dolezalek 
  pattern 
  with 
  a 
  plati- 
  

   num 
  suspension, 
  and 
  had 
  a 
  sensibility 
  of 
  140 
  mm 
  per 
  volt 
  with 
  

   84: 
  volts 
  on 
  the 
  needle. 
  

  

  In 
  most 
  of 
  the 
  experiments 
  a 
  potentiometer 
  arrangement 
  

   was 
  employed 
  so 
  that 
  the 
  reading 
  of 
  the 
  ionization 
  current 
  

   might 
  be 
  made 
  with 
  the 
  zero 
  of 
  the 
  instrument 
  at 
  the 
  center 
  

   of 
  the 
  swing. 
  It 
  was 
  found 
  that 
  this 
  method 
  gave 
  very 
  con- 
  

   sistent 
  and 
  accurate 
  readings, 
  which 
  was 
  of 
  especial 
  importance 
  

   when 
  small 
  potentials 
  were 
  employed. 
  

  

  The 
  whole 
  apparatus 
  could 
  be 
  rendered 
  gas-tight 
  by 
  the 
  use 
  

   of 
  a 
  heavy 
  stop-cock 
  grease. 
  

  

  Experimental 
  Results. 
  

  

  4. 
  In 
  the 
  first 
  set 
  of 
  observations 
  a 
  fine 
  slit 
  5 
  mm 
  long 
  and 
  

   0'5 
  mm 
  in 
  width 
  was 
  placed 
  l 
  cm 
  above 
  the 
  surface 
  of 
  the 
  polo- 
  

   nium. 
  A 
  i 
  Bragg' 
  curve 
  was 
  determined 
  in 
  the 
  usual 
  manner 
  

   by 
  varying 
  the 
  distance 
  of 
  the 
  polonium 
  from 
  the 
  electrodes. 
  

   The 
  range 
  of 
  the 
  alpha 
  particle 
  in 
  air 
  at 
  one 
  atmosphere 
  or 
  a 
  

   pressure 
  of 
  760 
  mm 
  of 
  mercury 
  was 
  found 
  to 
  be 
  3*8 
  cm 
  , 
  which 
  is 
  

   in 
  good 
  agreement 
  with 
  that 
  observed 
  by 
  Levin* 
  and 
  Taylorf. 
  

   When 
  the 
  transverse 
  field 
  was 
  employed 
  the 
  slit 
  was 
  so 
  orien- 
  

   tated 
  that 
  its 
  length 
  was 
  parallel 
  to 
  the 
  electrodes. 
  It 
  was 
  cal- 
  

  

  * 
  Levin, 
  this 
  Journal 
  (4), 
  xxii, 
  p. 
  8, 
  1906. 
  

   f 
  Taylor, 
  ibid. 
  (4), 
  xxvi, 
  p. 
  169, 
  1908. 
  

  

  