﻿On 
  a 
  Self-Recording 
  Electrometer. 
  

  

  201 
  

  

  Booms 
  suitably 
  counterpoised, 
  9J 
  in., 
  7-^ 
  in., 
  6| 
  in., 
  and 
  6 
  in. 
  

   long 
  were 
  tried 
  in 
  turn, 
  the 
  longest 
  being 
  used 
  first, 
  and 
  

   then 
  the 
  next 
  by 
  cutting 
  off 
  a 
  piece. 
  Joining 
  one 
  pair 
  of 
  

   quadrants 
  to 
  the 
  positive 
  terminal 
  of 
  an 
  80 
  volt 
  battery 
  and 
  the 
  

   other 
  to 
  the 
  negative, 
  the 
  centre 
  of 
  the 
  battery 
  being 
  earthed, 
  

   the 
  booms 
  gave 
  a 
  deflexion 
  of 
  1 
  millimetre 
  for 
  a 
  potential 
  of 
  

   12'6, 
  8, 
  (), 
  and 
  8*3 
  volts 
  on 
  the 
  needle 
  respectively. 
  Needles 
  

   4j 
  in., 
  4 
  in., 
  3J 
  in., 
  and 
  3 
  in. 
  in 
  diameter 
  were 
  also 
  tried 
  with 
  

   the 
  5£ 
  in. 
  and 
  6 
  in. 
  booms. 
  The 
  faces 
  of 
  the 
  quadrants 
  were 
  

   closer 
  together 
  for 
  the 
  smaller 
  than 
  for 
  the 
  larger 
  needles, 
  

   but 
  the 
  bifilar 
  suspension 
  was 
  the 
  same 
  in 
  all. 
  

   The 
  results 
  are 
  oiven 
  in 
  Table 
  I. 
  

  

  Table 
  I. 
  

  

  Length 
  of 
  

   Boom. 
  

  

  Yolts 
  on 
  needle 
  per 
  mm. 
  deflexion 
  with 
  P.D. 
  of 
  

   80 
  volts 
  across 
  quadrants. 
  

  

  4% 
  in. 
  needle. 
  

  

  4 
  in. 
  needle. 
  

  

  3^ 
  in. 
  needle. 
  

  

  3 
  in. 
  needle. 
  

  

  6^ 
  in 
  

  

  6 
  in 
  

  

  6 
  

  

  8-3 
  

  

  8'6 
  

   10 
  

  

  10 
  

   121 
  

  

  20 
  

   16-6 
  

  

  

  Both 
  sets 
  of 
  tests 
  show 
  that 
  the 
  6^- 
  in. 
  boom 
  and 
  the 
  4-L 
  in. 
  

   needle 
  give 
  the 
  best 
  results. 
  Owing 
  to 
  the 
  crude 
  con- 
  

   struction 
  of 
  the 
  instrument, 
  however, 
  it 
  is 
  not 
  possible 
  to 
  

   make 
  any 
  further 
  comparison. 
  

  

  If 
  the 
  vane 
  is 
  made 
  as 
  light 
  as 
  possible, 
  the 
  weight 
  of 
  the 
  

   boom, 
  its 
  counterpoise 
  and 
  the 
  attachment 
  to 
  the 
  needle, 
  is 
  

   very 
  much 
  greater 
  than 
  that 
  of 
  the 
  vane 
  of 
  the 
  needle, 
  so 
  that 
  

   the 
  sensitiveness 
  of 
  the 
  suspension 
  cannot 
  be 
  altered 
  appre- 
  

   ciably 
  for 
  the 
  smaller 
  vanes; 
  and 
  thus 
  we 
  get 
  the 
  greater 
  

   sensitiveness 
  for 
  the 
  larger 
  needles. 
  Owing 
  to 
  the 
  con- 
  

   struction 
  of 
  the 
  thread-recorder, 
  it 
  was 
  not 
  possible 
  without 
  

   greatly 
  increasing 
  the 
  length 
  of 
  the 
  boom 
  to 
  make 
  the 
  

   quadrants 
  larger 
  than 
  would 
  suit 
  a 
  4^ 
  in. 
  needle, 
  so 
  that 
  

   larger 
  needles 
  were 
  not 
  tried. 
  

  

  These 
  considerations 
  >uge;<>sted 
  that 
  if 
  a 
  second 
  vane 
  were 
  

   added 
  and 
  doubled-celled 
  quadrants 
  used, 
  the 
  weight 
  of 
  the 
  

   suspended 
  system 
  would 
  not 
  be 
  appreciably 
  altered, 
  and 
  

   the 
  deflecting 
  force 
  on 
  the 
  needle 
  would 
  be 
  doubled, 
  thus 
  

   almost 
  doubling 
  the 
  sensitiveness 
  of 
  the 
  instrument. 
  

  

  