﻿to 
  the 
  Measurement 
  of 
  Short 
  Electric 
  Waves. 
  255 
  

  

  cotton 
  wool 
  of 
  about 
  the 
  same 
  thickness; 
  and 
  the 
  whole 
  

   instrument 
  is 
  then 
  put 
  under 
  a 
  pile 
  of 
  felt 
  5 
  or 
  6 
  cm 
  thick. 
  

   The 
  only 
  part 
  that 
  is 
  at 
  all 
  exposed 
  is 
  the 
  mirror 
  of 
  the 
  suspen- 
  

   sion, 
  which 
  is 
  viewed 
  through 
  a 
  double 
  glass 
  window, 
  and 
  is 
  

   further 
  protected 
  by 
  screens 
  from 
  lamps 
  in 
  the 
  room 
  and 
  from 
  

   the 
  body 
  of 
  the 
  observer. 
  

  

  The 
  suspension 
  is 
  hung 
  by 
  a 
  fine 
  quartz 
  fiber, 
  of 
  which 
  the 
  

   force 
  of 
  torsion 
  serves 
  as 
  control. 
  The 
  weight 
  of 
  the 
  system 
  

   is 
  '27 
  gram, 
  of 
  which 
  the 
  mirror 
  weighs 
  '05, 
  and 
  the 
  resonating 
  

   cylinders 
  "144 
  gram. 
  The 
  period 
  of 
  a 
  swing 
  one 
  way 
  is 
  ?> 
  

   seconds. 
  The 
  resistance 
  of 
  the 
  thermal-junction 
  is 
  about 
  2 
  

   ohms. 
  

  

  It 
  might 
  be 
  supposed 
  that 
  the 
  sensitiveness 
  of 
  the 
  instrument 
  

   could 
  be 
  increased 
  by 
  making 
  the 
  coil 
  of 
  about 
  30 
  turns, 
  which 
  

   would 
  give 
  it 
  a 
  resistance 
  equal 
  to 
  that 
  of 
  the 
  thermal-junction. 
  

   I 
  have 
  found, 
  however, 
  that 
  the 
  control 
  by 
  the 
  field, 
  due 
  to 
  the 
  

   magnetic 
  properties 
  of 
  the 
  copper, 
  is 
  so 
  great 
  as 
  to 
  give 
  a 
  

   smaller 
  sensitiveness 
  with 
  a 
  large 
  number 
  of 
  turns 
  than 
  with 
  a 
  

   single 
  loop. 
  This 
  instrument, 
  because 
  of 
  the 
  resistance 
  of 
  the 
  

   thermal-juuction, 
  cannot 
  be 
  made 
  to 
  have 
  the 
  extreme 
  delicacy 
  

   of 
  Mr. 
  Boys's 
  original 
  radio-micrometer."* 
  It 
  has, 
  on 
  the 
  other 
  

   hand, 
  a 
  sensitiveness 
  that 
  is 
  not 
  easily 
  obtainable 
  by 
  the 
  use 
  of 
  

   a 
  thermal-junction 
  with 
  a 
  low 
  resistance 
  galvanometer, 
  as 
  is 
  

   shown 
  by 
  the 
  following 
  rough 
  test 
  : 
  

  

  .1 
  thermal 
  couple 
  of 
  German 
  silver 
  and 
  manganine 
  wires 
  of 
  

   commercial 
  size, 
  when 
  connected 
  to 
  a 
  galvanometer 
  with 
  a 
  

   figure 
  of 
  merit 
  of 
  10~ 
  8 
  , 
  gave 
  a 
  deflection 
  of 
  2 
  cm 
  when 
  heat 
  was 
  

   focussed 
  upon 
  it 
  from 
  a 
  distant 
  lamp. 
  The 
  instrument 
  de- 
  

   scribed 
  above 
  with 
  approximately 
  the 
  same 
  heat, 
  less 
  the 
  

   amount 
  absorbed 
  by 
  the 
  glass 
  tube, 
  gave 
  a 
  deflection 
  of 
  40 
  cm 
  ; 
  

   to 
  obtain 
  which 
  the 
  German-silver-inanganine 
  couple 
  would 
  

   have 
  required 
  a 
  galvanometer 
  capable 
  of 
  giving 
  a 
  deflection 
  of 
  

   l 
  mm 
  , 
  at 
  a 
  scale 
  distance 
  of 
  100 
  cm 
  , 
  for 
  a 
  current 
  of 
  5 
  X 
  10" 
  10 
  

   amperes. 
  

  

  Another 
  advantage 
  of 
  the 
  D'Arsonval 
  arrangement 
  is 
  that 
  it 
  

   can 
  practically 
  be 
  made 
  dead-beat 
  and 
  is 
  unaffected 
  by 
  outside 
  

   magnetic 
  disturbances. 
  This 
  is 
  important 
  in 
  the 
  measurement 
  

   of 
  electric 
  waves, 
  since, 
  on 
  account 
  of 
  the 
  variable 
  nature 
  of 
  

   the 
  source 
  of 
  the 
  waves, 
  throws 
  instead 
  of 
  permanent 
  deflec- 
  

   tions 
  must 
  be 
  read; 
  and 
  to 
  get 
  consistent 
  throws 
  it 
  is 
  necessary 
  

   to 
  have 
  the 
  suspended 
  system 
  start 
  from 
  a 
  state 
  of 
  rest, 
  a 
  con- 
  

   dition 
  difficult 
  to 
  attain 
  with 
  a 
  sensitive 
  galvanometer 
  even 
  

   when 
  shielded. 
  The 
  instrument 
  described 
  above 
  has 
  kept 
  its 
  

   zero 
  within 
  a 
  very 
  small 
  range 
  for 
  over 
  a 
  month, 
  and 
  after 
  a 
  

   reading 
  is 
  taken 
  comes 
  rapidly 
  to 
  rest, 
  disturbed 
  only 
  by 
  the 
  

   slight 
  tremors 
  of 
  the 
  pier 
  on 
  which 
  it 
  is 
  placed. 
  

  

  *C. 
  V. 
  Boys: 
  Phil. 
  Tran?., 
  vol. 
  clxxx, 
  A, 
  p. 
  159, 
  1888-1889. 
  

  

  