﻿848 
  Mr. 
  ¥:^oddj 
  ontJie 
  

  

  globe 
  A 
  of 
  about 
  half 
  a 
  litre 
  capacity, 
  provided 
  with 
  a 
  circular 
  

   plane 
  parallel 
  plate 
  W 
  to 
  serve 
  as 
  a 
  window, 
  and 
  silvered 
  

  

  Fig. 
  2. 
  

  

  internally 
  except 
  opposite 
  the 
  window. 
  The 
  window 
  greatly 
  

   improves 
  the 
  sharpness 
  of 
  the 
  image 
  in 
  the 
  microscope, 
  and 
  

   the 
  accuracy 
  of 
  the 
  readings. 
  C, 
  D 
  are 
  circles 
  of 
  aluminium- 
  

   foil 
  to 
  act 
  as 
  baffle-plates, 
  which 
  are 
  pasted 
  over 
  the 
  inside 
  

   at 
  the 
  inlet 
  and 
  outlet 
  taps 
  B 
  and 
  G. 
  The 
  fine 
  thermometer- 
  

   tube 
  E 
  also 
  assists 
  in 
  moderating 
  the 
  disturbance 
  of 
  the 
  leaf 
  

   by 
  the 
  entering 
  air. 
  The 
  electroscope 
  is 
  kept 
  exhausted 
  and 
  

   charged 
  when 
  not 
  in 
  use 
  during 
  any 
  series 
  of 
  successive 
  

   measurements. 
  Its 
  normal 
  natural 
  leak 
  has 
  been 
  constant 
  

   within 
  10 
  per 
  cent, 
  at 
  1*1 
  units 
  since 
  it 
  was 
  constructed. 
  

   This 
  is 
  first 
  determined 
  accurately. 
  Several 
  small 
  quantities 
  

   of 
  air 
  are 
  admitted 
  and 
  pumped 
  out, 
  before 
  finally 
  filling 
  the 
  

   instrument 
  with 
  air. 
  The 
  natural 
  leak 
  is 
  then 
  taken 
  over 
  

   several 
  hours, 
  prior 
  to 
  the 
  admission 
  of 
  the 
  emanation. 
  

   When 
  this 
  has 
  been 
  done 
  the 
  instrument 
  is 
  exhausted 
  and 
  

   the 
  calcium-chloride 
  tube 
  K 
  connected 
  to 
  the 
  tube 
  D 
  of 
  

   fig. 
  1. 
  When 
  the 
  whole 
  of 
  the 
  gas 
  in 
  G 
  has 
  been 
  drawn 
  in, 
  

   D 
  is 
  disconnected 
  and 
  fresh 
  air 
  allowed 
  to 
  enter 
  to 
  atmo- 
  

   spheric 
  pressure. 
  No 
  trace 
  of 
  emanation 
  is 
  thus 
  lost. 
  The 
  

   leak 
  is 
  taken 
  over 
  the 
  period 
  from 
  8 
  to 
  16 
  minutes 
  after 
  the 
  

   admission 
  of 
  the 
  emanation, 
  and 
  again 
  after 
  the 
  lapse 
  of 
  

   three 
  hours. 
  The 
  leaf 
  is 
  kept 
  continuously 
  charged 
  (nega- 
  

   tively) 
  with 
  the 
  charging-rod 
  earthed 
  over 
  the 
  whole 
  interval. 
  

   This 
  is 
  important, 
  as 
  otherwise 
  a 
  considerable 
  error 
  is 
  intro- 
  

   duced, 
  which 
  does 
  not 
  appear 
  to 
  have 
  been 
  previously 
  pointed 
  

   out. 
  The 
  error 
  just 
  referred 
  to 
  applies 
  particularly 
  to 
  the 
  

   calibration 
  of 
  the 
  electroscope. 
  With 
  the 
  minute 
  quantities 
  

   of 
  radium 
  present 
  in 
  the 
  uranium 
  solutions, 
  the 
  leaf 
  if 
  fully 
  

   charged 
  initially 
  remains 
  charged 
  over 
  the 
  three-hour 
  

   interval, 
  and 
  the 
  practice 
  throughout 
  since 
  this 
  method 
  of 
  

   measurement 
  was 
  adopted 
  has 
  been 
  to 
  keep 
  it 
  charged 
  during 
  

   the 
  whole 
  interval. 
  With 
  the 
  radium 
  standards, 
  however, 
  

  

  