﻿Self-Recording 
  Electromeh 
  r. 
  209 
  

  

  boom 
  and 
  ir> 
  counterpoise, 
  and 
  the 
  weight 
  of 
  the 
  vane 
  can 
  be 
  

   neglected, 
  so 
  that 
  by 
  doubling 
  the 
  area 
  of 
  the 
  needle 
  the 
  de- 
  

   flecting 
  force 
  would 
  be 
  doubled, 
  and 
  the 
  sensitiveness 
  of 
  the 
  

   instrument 
  would 
  be 
  doubled 
  also, 
  provided 
  the 
  boom 
  and 
  its 
  

   counterpoise 
  were 
  the 
  same 
  as 
  before. 
  Now 
  as 
  the 
  boom 
  

   and 
  counterpoise 
  are 
  the 
  chief 
  factors 
  in 
  limiting 
  the 
  sensi- 
  

   tiveness 
  of 
  the 
  instrument, 
  it 
  might 
  be 
  assumed 
  that 
  it 
  would 
  

   have 
  been 
  much 
  simpler 
  to 
  have 
  increased 
  the 
  size 
  of 
  the 
  

   needle 
  and 
  obtained 
  the 
  same 
  result 
  in 
  that 
  way. 
  The 
  pre- 
  

   liminary 
  experiments 
  indicated 
  that 
  the 
  6-^ 
  in. 
  boom 
  was 
  the 
  

   best. 
  Having 
  used 
  the 
  best 
  length 
  of 
  boom 
  and 
  made 
  the 
  

   diameter 
  of 
  the 
  quadrants 
  as 
  great 
  as 
  possible 
  for 
  this 
  length 
  

   of 
  boom, 
  the 
  best 
  arrangement 
  of 
  boom 
  and 
  needle 
  was 
  

   obtained. 
  Any 
  further 
  increase 
  in 
  the 
  needle 
  would 
  have 
  

   necessitated 
  a 
  longer 
  boom, 
  and 
  owing 
  to 
  the 
  additional 
  

   weight 
  would 
  have 
  required 
  a 
  stronger 
  suspension, 
  thus 
  

   largely 
  counteracting 
  an}- 
  gain 
  obtained 
  by 
  the 
  increased 
  

   size 
  of 
  the 
  needle. 
  There 
  remained 
  therefore 
  the 
  method, 
  

   which 
  was 
  adopted, 
  of 
  double 
  decking 
  the 
  quadrants 
  and 
  

   using 
  a 
  two-vaned 
  needle, 
  to 
  obtain 
  the 
  greater 
  sensitiveness. 
  

   Another 
  question 
  that 
  arises 
  is 
  as 
  to 
  whether 
  an 
  instrument 
  

   that 
  requires 
  a 
  battery 
  of 
  a 
  few 
  volts 
  can 
  be 
  kept 
  as 
  constant 
  

   as 
  a 
  battery 
  of 
  100 
  volts 
  or 
  more. 
  In 
  this 
  instrument 
  a 
  P.D. 
  

   of 
  12 
  volts 
  across 
  the 
  quadrants 
  is 
  sufficient 
  for 
  measuring 
  

   the 
  potential 
  of 
  the 
  air; 
  a 
  reference 
  to 
  fig. 
  4 
  will 
  show 
  that 
  

   a 
  change 
  of 
  1 
  per 
  cent, 
  in 
  the 
  voltage 
  will 
  cause 
  a 
  change 
  of 
  

   about 
  1 
  per 
  cent, 
  in 
  the 
  readings 
  of 
  the 
  instrument. 
  A 
  

   change 
  of 
  '5 
  volt 
  in 
  a 
  battery 
  of 
  12 
  volts 
  would 
  be 
  large 
  

   when 
  no 
  current 
  is 
  being 
  used, 
  yet 
  this 
  would 
  not 
  alter 
  the 
  

   value 
  of 
  the 
  readings 
  by 
  more 
  than 
  4 
  per 
  cent. 
  

  

  The 
  advantages 
  claimed 
  for 
  the 
  instrument 
  are 
  : 
  — 
  

  

  1. 
  Simplicity 
  of 
  construction. 
  

  

  2. 
  Little 
  attention 
  required 
  when 
  in 
  working 
  condition. 
  

  

  3. 
  High 
  degree 
  of 
  sensibility 
  enabling 
  the 
  potential 
  of 
  

  

  the 
  air 
  to 
  be 
  measured 
  with 
  a 
  battery 
  of 
  only 
  a 
  few 
  

   volts. 
  

  

  4. 
  Fair 
  uniformity 
  of 
  scale 
  over 
  the 
  range 
  of 
  the 
  recording 
  

  

  paper. 
  

  

  Phil. 
  Mag. 
  8. 
  6. 
  Vol. 
  26. 
  No. 
  151. 
  Jabj 
  1913. 
  

  

  