﻿742 
  Prof. 
  McLennan 
  and 
  Mr. 
  McLeod: 
  Measurements 
  

  

  on 
  

  

  system 
  consisted 
  of 
  two 
  conducting 
  fused-quartz 
  fibres 
  

   attached 
  at 
  their 
  upper 
  ends 
  to 
  an 
  insulating 
  support 
  and 
  at 
  

   their 
  lower 
  ends 
  to 
  a 
  cross 
  fibre 
  also 
  of 
  fused 
  quartz 
  under 
  

   tension. 
  This 
  cross 
  fibre 
  was 
  attached 
  to 
  an 
  insulating 
  

   support 
  as 
  shown 
  in 
  the 
  figure. 
  The 
  instrument 
  was 
  pro- 
  

   vided 
  with 
  a 
  metal 
  tube 
  ss, 
  which 
  could 
  be 
  slid 
  down 
  over 
  

   the 
  electrical 
  system. 
  The 
  use 
  of 
  this 
  sliding 
  tube 
  was 
  to 
  

   limit 
  the 
  volume 
  from 
  which 
  ions 
  could 
  be 
  drawn 
  to 
  the 
  

   fibres 
  to 
  as 
  low 
  a 
  value 
  as 
  possible. 
  With 
  the 
  tube 
  raised 
  

   the 
  effective 
  volume 
  as 
  given 
  by 
  the 
  makers 
  was 
  2021*1 
  c.c, 
  

   but 
  with 
  it 
  lowered 
  the 
  effective 
  volume 
  was 
  only 
  31'5 
  c.c. 
  

  

  The 
  fibres 
  were 
  illuminated 
  by 
  light 
  thrown 
  into 
  the 
  

   instrument 
  by 
  a 
  mirror 
  through 
  a 
  window, 
  and 
  the 
  readings 
  

   were 
  taken 
  with 
  a 
  microscope 
  provided 
  with 
  a 
  scale. 
  The 
  

   air 
  in 
  the 
  receiver 
  was 
  kept 
  dry 
  with 
  a 
  small 
  quantity 
  of 
  

   sodium 
  carried 
  in 
  a 
  small 
  receptacle 
  attached 
  to 
  the 
  body 
  

   of 
  the 
  instrument. 
  The 
  electrical 
  system 
  in 
  practice 
  was 
  

   charged 
  by 
  means 
  of 
  a 
  Zamboni 
  pile 
  through 
  the 
  intermediary 
  

   of 
  an 
  insulated 
  metallic 
  sound 
  which 
  passed 
  through 
  the 
  case, 
  

   and 
  which 
  could 
  be 
  turned 
  when 
  desired 
  so 
  as 
  to 
  come 
  into 
  

   contact 
  with 
  a 
  metal 
  piece 
  which 
  connected 
  the 
  fibres 
  to 
  

   their 
  upper 
  insulating 
  support. 
  All 
  the 
  openings 
  into 
  the 
  

   Teceiver, 
  with 
  the 
  exception 
  of 
  the 
  covering 
  tube 
  A, 
  were 
  

   -either 
  closed 
  by 
  soldered 
  contacts 
  or 
  by 
  fluted 
  joints 
  pro- 
  

   vided 
  with 
  leather 
  washers. 
  The 
  lower 
  end 
  of 
  the 
  covering 
  

   tube 
  A 
  had 
  a 
  ground 
  conical 
  surface, 
  and 
  when 
  in 
  position 
  

   was 
  held 
  hermetically 
  connected 
  to 
  the 
  body 
  of 
  the 
  instru- 
  

   ment 
  with 
  a 
  shoulder 
  piece 
  provided 
  with 
  a 
  thread. 
  The 
  

   instrument 
  was 
  provided 
  with 
  a 
  covering 
  tube 
  for 
  the 
  micro- 
  

   scops 
  and 
  a 
  cover 
  plate 
  for 
  the 
  window. 
  Both 
  of 
  these 
  

   could 
  be 
  screwed 
  tightly 
  against 
  leather 
  washers. 
  

  

  When 
  all 
  the 
  openings 
  into 
  the 
  receiver 
  were 
  carefully 
  

   closed 
  the 
  instrument 
  was 
  both 
  water-tight 
  and 
  air-tight. 
  

  

  The 
  electrical 
  system 
  was 
  calibrated 
  in 
  the 
  ordinary 
  way, 
  

   and 
  by 
  means 
  of 
  a 
  calibration 
  curve 
  the 
  readings 
  were 
  

   transcribed 
  from 
  changes 
  in 
  deflexions 
  into 
  potential 
  falls. 
  

  

  The 
  capacity 
  of 
  the 
  electrical 
  system 
  was 
  1*04 
  cm. 
  

  

  The 
  theory 
  of 
  the 
  instrument 
  is 
  as 
  follows 
  : 
  — 
  

  

  Let 
  Q 
  be 
  the 
  loss 
  of 
  charge 
  observed 
  in 
  one 
  second 
  and 
  A 
  

   the 
  part 
  of 
  Q 
  due 
  to 
  the 
  leak 
  over 
  the 
  insulating 
  

   supports. 
  

  

  Let 
  V 
  be 
  the 
  effective 
  volume 
  with 
  the 
  tube 
  ss 
  up, 
  and 
  

   ~V 
  l 
  its 
  effective 
  volume 
  with 
  the 
  tube 
  down. 
  

  

  Then 
  Q 
  = 
  A 
  + 
  BV 
  where 
  B 
  is 
  a 
  constant, 
  

  

  and 
  Q 
  ] 
  =A 
  + 
  BV 
  1 
  , 
  

  

  