﻿202 
  Mr. 
  J. 
  Patterson 
  on 
  a 
  

  

  In 
  testing 
  this 
  the 
  results 
  given 
  in 
  Table 
  II. 
  were 
  obtained. 
  

  

  Table 
  II. 
  

  

  Length 
  

  

  of 
  

   Boom. 
  

  

  6£in.... 
  

   6 
  in. 
  ... 
  

  

  Volts 
  on 
  needle 
  per 
  mm. 
  deflexion 
  with 
  P.D. 
  of 
  80 
  volts 
  

   across 
  quadrants. 
  

  

  4i 
  in. 
  needle. 
  

  

  4 
  in. 
  needle. 
  

  

  3£ 
  in. 
  needle. 
  

  

  3 
  in. 
  needle. 
  

  

  Single. 
  

  

  Double. 
  

  

  Single. 
  

  

  Double. 
  

  

  Single. 
  

  

  Double. 
  

  

  Single. 
  

  

  Double. 
  

  

  6 
  

   8-3 
  

  

  4 
  

   4-4 
  

  

  8-6 
  

   10 
  

  

  6-3 
  

   45 
  

  

  10 
  

   12-1 
  

  

  71 
  

  

  6-8 
  

  

  20-0 
  

   16-6 
  

  

  8-6 
  

   8-2 
  

  

  There 
  is 
  thus 
  a 
  decided 
  gain 
  in 
  the 
  sensitiveness 
  by 
  using 
  

   two 
  vanes; 
  but, 
  as 
  before 
  mentioned^ 
  the 
  results 
  are 
  only 
  

   indicative 
  of 
  what 
  might 
  be 
  expected 
  in 
  the 
  finished 
  in- 
  

   strument, 
  and 
  cannot 
  be 
  used 
  for 
  comparative 
  purposes. 
  It 
  

   may 
  not 
  be 
  out 
  of 
  place 
  here 
  to 
  mention 
  that 
  on 
  testing 
  the 
  

   openness 
  of 
  scale 
  with 
  one 
  vane 
  and 
  with 
  two 
  vanes 
  in 
  

   the 
  finished 
  instrument, 
  the 
  former 
  gave 
  a 
  deflexion 
  of 
  

   11 
  volts 
  per 
  mm. 
  under 
  the 
  same 
  conditions 
  that 
  the 
  latter 
  

   gave 
  5*2 
  volts 
  per 
  mm., 
  showing 
  that 
  the 
  double-vaned 
  

   needle 
  gives 
  double 
  the 
  openness 
  of 
  scale. 
  

  

  As 
  a 
  result 
  of 
  the 
  preliminary 
  tests 
  the 
  electrometer 
  was 
  

   constructed 
  with 
  a 
  needle 
  having 
  two 
  vanes 
  4i 
  in. 
  in 
  diameter, 
  

   and 
  a 
  boom 
  6J 
  in. 
  long. 
  

  

  Description 
  of 
  the 
  Instrument, 
  

  

  Plate 
  VI. 
  fig. 
  1 
  is 
  a 
  photograph 
  of 
  the 
  needle 
  system 
  

   with 
  the 
  boom 
  attached. 
  The 
  vanes 
  of 
  the 
  needle 
  are 
  made 
  

   of 
  aluminium 
  0*085 
  mm. 
  thick, 
  and 
  in 
  order 
  to 
  give 
  the 
  vanes 
  

   rigidity 
  the 
  edges 
  are 
  slightly 
  turned 
  up 
  as 
  shown 
  in 
  the 
  

   photograph. 
  

  

  In 
  order 
  to 
  render 
  the 
  instrument 
  fairly 
  dead-beat 
  it 
  is 
  

   necessary 
  to 
  have 
  a 
  strong 
  damping 
  attachment, 
  but 
  at 
  the 
  

   same 
  time 
  it 
  was 
  felt 
  that 
  a 
  system 
  that 
  would 
  do 
  away 
  with 
  

   the 
  usual 
  dashpot 
  arrangement 
  of 
  a 
  platinum 
  vane 
  swinging 
  

   in 
  a 
  jar 
  of 
  acid, 
  with 
  the 
  surface 
  tension 
  effects 
  on 
  the 
  needle 
  

   and 
  the 
  usual 
  troubles 
  incident 
  to 
  this 
  method, 
  was 
  much 
  to 
  

   be 
  desired. 
  This 
  was 
  accomplished 
  satisfactorily 
  by 
  the 
  

   Cambridge 
  Scientific 
  Instrument 
  Co. 
  by 
  attaching 
  to 
  the 
  

   rod 
  oi: 
  the 
  needle 
  a 
  very 
  light 
  circular 
  vane 
  which 
  swings 
  

   between 
  the 
  poles 
  of 
  two 
  strong 
  permanent 
  magnets, 
  as 
  

   shown 
  in 
  Plate 
  VI. 
  fig. 
  1. 
  These 
  magnets 
  can 
  be 
  adjusted 
  

   separately 
  to 
  suit 
  the 
  position 
  of 
  the 
  vane. 
  

  

  