﻿the 
  Reflexion 
  of 
  the 
  X-rays. 
  215 
  

  

  maintained 
  in 
  the 
  gas 
  which 
  was 
  just 
  not 
  sufficient 
  to 
  cause 
  

   the 
  passage 
  of 
  a 
  spark. 
  Townsend 
  has 
  shown 
  that 
  both 
  

   positive 
  and 
  negative 
  ions 
  then 
  acquire 
  sufficient 
  speed 
  to 
  

   produce 
  fresh 
  ionization, 
  and 
  that 
  the 
  current 
  through 
  the 
  

   gas 
  is 
  in 
  this 
  way 
  multiplied 
  by 
  a 
  large 
  but 
  constant 
  factor. 
  

   This 
  method 
  was 
  first 
  employed 
  by 
  Griffith 
  *, 
  who 
  used 
  it 
  in 
  

   quantitative 
  measurements 
  of 
  the 
  photoelectric 
  effect. 
  

   Rutherford 
  and 
  Geiger 
  t 
  in 
  their 
  electrical 
  method 
  of 
  

   counting 
  a 
  particles 
  were 
  the 
  first 
  to 
  use 
  helium. 
  

  

  The 
  JJetector. 
  — 
  The 
  detector 
  was 
  a 
  gas-tight 
  brass 
  box 
  

   1" 
  cm. 
  long, 
  2 
  cm. 
  broad, 
  and 
  4i 
  cm. 
  deep, 
  fixed 
  to 
  the 
  

   telescope-arm 
  by 
  an 
  ebonite 
  block. 
  It 
  is 
  shown 
  in 
  section 
  

   in 
  fig. 
  1 
  B. 
  The 
  rays 
  entered 
  *at 
  one 
  end 
  by 
  a 
  thin 
  mica, 
  

   window 
  m 
  surrounded 
  by 
  a 
  lead 
  screen 
  r. 
  They 
  fell 
  at 
  a 
  

   very 
  oblique 
  angle 
  on 
  the 
  plate 
  c. 
  This 
  plate 
  was 
  one 
  side 
  

   of" 
  the 
  box. 
  which 
  was 
  kept 
  charged 
  to 
  a 
  potential 
  of 
  about 
  

   800 
  volts. 
  A 
  plate 
  d 
  accurately 
  parallel 
  to 
  c 
  and 
  2*8 
  mm. 
  

   from 
  it 
  was 
  connected 
  to 
  an 
  electrometer. 
  The 
  connecting 
  

   wire, 
  which 
  was 
  insulated 
  from 
  the 
  box 
  by 
  ebonite 
  plugs 
  

   and 
  a 
  guard-ring, 
  passed 
  to 
  the 
  axis 
  tube 
  of 
  the 
  spectrometer, 
  

   and 
  down 
  it 
  to 
  the 
  electrometer. 
  It 
  was 
  therefore 
  not 
  

   disturbed 
  by 
  moving 
  the 
  detector. 
  

  

  Hie 
  Helium. 
  — 
  Ionization 
  by 
  collision 
  may 
  be 
  obtained 
  by 
  

   the 
  use 
  of 
  helium 
  at 
  a 
  very 
  much 
  higher 
  pressure 
  for 
  a 
  given 
  

   potential 
  difference 
  than 
  is 
  possible 
  with 
  air. 
  The 
  great 
  

   ease 
  with 
  which 
  helium 
  may 
  be 
  purified 
  is 
  also 
  an 
  advantage. 
  

   It 
  was 
  purified 
  by 
  passing 
  it 
  over 
  charcoal 
  cooled 
  in 
  liquid 
  

   air, 
  and 
  it 
  was 
  then 
  pumped 
  into 
  the 
  detector, 
  which 
  was 
  

   temporarily 
  connected 
  to 
  the 
  purifying 
  apparatus 
  by 
  a 
  glass 
  

   tube 
  fitted 
  with 
  ground 
  joints. 
  It 
  was 
  found 
  advisable 
  to 
  

   clean 
  the 
  helium 
  every 
  few 
  weeks, 
  since 
  the 
  natural 
  ioniza- 
  

   tion 
  in 
  the 
  detector, 
  at 
  first 
  very 
  small, 
  increased 
  markedly 
  

   after 
  some 
  time. 
  This 
  curious 
  effect 
  suggests 
  that, 
  as 
  in 
  the 
  

   stagnant 
  air 
  of 
  cellars, 
  radium 
  emanation 
  gradually 
  diffused 
  

   from 
  the 
  walls 
  of 
  the 
  box. 
  The 
  properties 
  of 
  pure 
  helium 
  

   are 
  affected 
  to 
  an 
  extraordinarily 
  marked 
  degree 
  by 
  the 
  

   presence 
  of 
  traces 
  of 
  impurity. 
  It 
  was 
  therefore 
  not 
  sur- 
  

   prising 
  that 
  when 
  the 
  apparatus 
  was 
  first 
  used, 
  the 
  facto'- 
  by 
  

   which 
  a 
  given 
  difference 
  of 
  potential 
  increased 
  the 
  ionization 
  

   current 
  was 
  found 
  to 
  fall 
  off 
  very 
  rapidly, 
  owing 
  to 
  the 
  

   contamination 
  of 
  the 
  helium 
  by 
  traces 
  of 
  other 
  gases 
  coming 
  

   from 
  the 
  walls 
  of 
  the 
  box. 
  This 
  trouble 
  was 
  overcome 
  by 
  

   osing 
  helium 
  mixed 
  with 
  a 
  trace 
  of 
  air. 
  The 
  sparking 
  

   potential 
  was 
  in 
  this 
  way 
  raised 
  by 
  about 
  a 
  hundred 
  volts, 
  

  

  * 
  Griffith, 
  Phil. 
  Mag. 
  xiv. 
  p. 
  297 
  (1007). 
  

  

  t 
  Geiger 
  and 
  Putherforcl, 
  Phil. 
  Mag. 
  xxiv. 
  ]>. 
  618 
  (1012). 
  

  

  