﻿Horizontal 
  Intensity 
  of 
  the 
  Earth's 
  Magnetic 
  Field. 
  761 
  

  

  means 
  of 
  a 
  sliding 
  contact, 
  which 
  allows 
  of 
  a 
  minute 
  altera- 
  

   tion 
  in 
  the 
  magnitude 
  of 
  the 
  current 
  being 
  made. 
  

  

  The 
  standard 
  10 
  ohms 
  resistance 
  is 
  one 
  by 
  the 
  Cambridge 
  

   Scientific 
  Instrument 
  Co., 
  and 
  is 
  provided 
  with 
  a 
  calibration 
  

   curve 
  for 
  alteration 
  in 
  resistance 
  owing 
  to 
  change 
  of 
  tem- 
  

   perature. 
  The 
  resistance 
  itself 
  is 
  in 
  oil 
  contained 
  in 
  a 
  small 
  

   vessel 
  which 
  was 
  kept 
  in 
  a 
  bath 
  of 
  water 
  at 
  the 
  temperature 
  

   of 
  the 
  room. 
  Thick 
  copper 
  leads 
  dipping 
  into 
  mercury 
  cups 
  

   serve 
  as 
  a 
  means 
  of 
  connecting 
  the 
  resistance 
  with 
  the 
  other 
  

   apparatus. 
  The 
  actual 
  magnitude 
  of 
  the 
  current 
  is 
  deter- 
  

   mined 
  by 
  measuring 
  the 
  drop 
  in 
  potential 
  between 
  the 
  

   terminals 
  of 
  the 
  standard 
  resistance. 
  

  

  The 
  upper 
  portion 
  of 
  the 
  diagram 
  shows 
  the 
  method 
  of 
  

   measuring 
  this 
  drop 
  in 
  potential. 
  The 
  two 
  post-office 
  boxes- 
  

   G 
  x 
  G 
  2 
  form 
  a 
  potentiometer 
  to 
  the 
  terminals 
  of 
  which 
  a 
  

   Leclanche 
  cell 
  F 
  is 
  connected. 
  The 
  cadmium 
  cell 
  V 
  serves 
  

   to 
  standardize 
  the 
  Leclanche, 
  the 
  voltage 
  of 
  which 
  is 
  constant,, 
  

   for 
  the 
  total 
  resistance 
  in 
  the 
  post-office 
  boxes 
  never 
  varies 
  

   by 
  more 
  than 
  1 
  or 
  2 
  ohms 
  from 
  10,000 
  ohms. 
  

  

  H 
  is 
  a 
  sensitive 
  mirror-galvanometer 
  into 
  the 
  circuit 
  of 
  

   which 
  a 
  resistance 
  J 
  can 
  be 
  switched 
  in, 
  or 
  out, 
  at 
  will 
  by 
  

   means 
  of 
  a 
  three-w 
  7 
  ay 
  key. 
  

  

  The 
  terminals 
  of 
  the 
  standard 
  10 
  ohms 
  resistance 
  were 
  

   connected 
  to 
  the 
  post-office 
  box 
  potentiometer 
  as 
  shown. 
  

  

  The 
  following 
  is 
  the 
  method 
  of 
  making 
  the 
  measurement 
  

   of 
  the 
  current 
  round 
  the 
  solenoid. 
  

  

  The 
  high 
  resistance 
  J 
  and 
  the 
  cadmium 
  cell 
  are 
  switched 
  

   into 
  the 
  galvanometer 
  circuit, 
  and 
  the 
  resistance 
  in 
  G 
  } 
  is 
  

   altered 
  until 
  no 
  deflexion 
  of 
  the 
  galvanometer 
  mirror 
  takes 
  

   place. 
  Whatever 
  alteration 
  in 
  the 
  resistance 
  of 
  G 
  is 
  made 
  

   is 
  compensated 
  for 
  by 
  altering 
  G 
  x 
  2 
  so 
  that 
  resistance 
  in 
  G 
  1 
  

   plus 
  resistance 
  in 
  G' 
  2 
  always 
  equals 
  10,000. 
  The 
  high 
  resist- 
  

   ance 
  J 
  is 
  now 
  switched 
  out 
  of 
  the 
  galvanometer 
  circuit 
  and 
  

   a 
  more 
  delicate 
  observation 
  made. 
  It 
  may 
  not 
  be 
  possible 
  

   to 
  get 
  an 
  exact 
  balance 
  by 
  keeping 
  the 
  sum 
  G+G' 
  = 
  10,000, 
  

   and 
  in 
  this 
  case 
  G 
  + 
  G' 
  is 
  altered 
  very 
  slightly, 
  say 
  to 
  10,002, 
  

   so 
  that 
  accuracy 
  may 
  be 
  ensured. 
  

  

  The 
  cadmium 
  cell 
  is 
  now 
  switched 
  out, 
  and 
  the 
  drop 
  in 
  

   potential 
  between 
  the 
  terminals 
  of 
  the 
  standard 
  10 
  ohms 
  

   resistance 
  switched 
  in. 
  The 
  previous 
  process 
  is 
  now 
  

   repeated 
  until 
  a 
  balance 
  is 
  obtained. 
  The 
  calculation 
  of 
  the 
  

   drop 
  in 
  potential 
  between 
  the 
  terminals 
  of 
  the 
  standard 
  

   resistance 
  is 
  simple. 
  The 
  cadmium 
  cell 
  was 
  immersed 
  in 
  

   water 
  to 
  keep 
  its 
  temperature 
  constant, 
  and 
  as 
  the 
  makers 
  

   provided 
  a 
  temperature 
  calibration 
  curve, 
  its 
  E.M.F. 
  is 
  

   known 
  accurately. 
  Immediately 
  after 
  the 
  experiments 
  it 
  

   was 
  sent 
  to 
  the 
  National 
  Physical 
  Laboratory 
  to 
  be 
  tested. 
  

  

  