﻿208 
  Mr. 
  J. 
  Patterson 
  on 
  a 
  

  

  an 
  area 
  of 
  high 
  pressure 
  was 
  approaching, 
  giving 
  very 
  frosty 
  

   weather 
  and 
  fairly 
  clear 
  skies. 
  

  

  Fig. 
  5. 
  

  

  /A, 
  

  

  X 
  » 
  

  

  W^ 
  marks 
  

  

  Conclusion. 
  

  

  The 
  question 
  naturally 
  arises 
  whether 
  it 
  is 
  better 
  to 
  make 
  

   a 
  needle 
  with 
  two 
  vanes 
  rather 
  than 
  an 
  equivalent 
  single 
  

   vaned 
  needle. 
  

  

  If 
  one 
  increased 
  the 
  sensitiveness 
  by 
  increasing 
  the 
  size 
  

   of 
  the 
  needle, 
  the 
  best 
  that 
  it 
  would 
  be 
  possible 
  to 
  do 
  would 
  

   be 
  to 
  make 
  an 
  instrument 
  that 
  had 
  its 
  plates 
  the 
  same 
  distance 
  

   apart 
  for 
  the 
  larger 
  as 
  for 
  the 
  smaller 
  needle, 
  and 
  that 
  had 
  

   the 
  needles 
  constructed 
  of 
  material 
  of 
  the 
  same 
  gauge. 
  In 
  

   this 
  case, 
  if 
  the 
  linear 
  dimension 
  of 
  the 
  needle 
  were 
  multiplied 
  

   by 
  //,, 
  the 
  capacity 
  would 
  be 
  multiplied 
  by 
  jj?. 
  The 
  surface 
  

   density 
  for 
  the 
  same 
  voltage 
  would 
  not 
  change, 
  nor 
  the 
  

   electro-mechanical 
  force 
  per 
  unit 
  area, 
  nor 
  the 
  moment 
  

   about 
  the 
  suspending 
  wire 
  of 
  the 
  electric 
  force 
  per 
  unit 
  area. 
  

   The 
  total 
  moment 
  would 
  then 
  be 
  increased 
  by 
  fju 
  2 
  , 
  or 
  the 
  de- 
  

   flecting 
  force 
  on 
  the 
  needle 
  would 
  be 
  multiplied 
  by 
  the 
  increase 
  

   of 
  the 
  area 
  of 
  the 
  needle. 
  Thus 
  if 
  the 
  area 
  of 
  the 
  needle 
  were 
  

   doubled 
  the 
  weight 
  would 
  be 
  doubled, 
  and 
  the 
  deflecting 
  

   force 
  doubled. 
  This 
  is 
  the 
  best 
  that 
  it 
  would 
  be 
  possible 
  to 
  

   obtain. 
  If 
  it 
  is 
  assumed 
  that 
  it 
  is 
  possible 
  to 
  obtain 
  this 
  con- 
  

   dition, 
  would 
  it 
  give 
  an 
  instrument 
  as 
  sensitive 
  as 
  one 
  obtained 
  

   by 
  adding 
  two 
  vanes 
  and 
  using 
  double-celled 
  quadrants 
  ? 
  

   Almost 
  the 
  whole 
  weight 
  of 
  the 
  suspended 
  system 
  is 
  in 
  the 
  

  

  