﻿178 
  

  

  Mr. 
  W. 
  Duddell 
  on 
  a 
  

  

  The 
  very 
  large 
  amount 
  that 
  the 
  apparent 
  resistance 
  o£ 
  the 
  

   instrument 
  is 
  increased 
  by 
  its 
  back 
  E.M.F., 
  over 
  200 
  ohms 
  

   in 
  one 
  case, 
  shows 
  how 
  very 
  important 
  it 
  is 
  to 
  try 
  to 
  keep 
  

  

  Fig. 
  10. 
  

  

  200 
  

  

  

  

  

  

  

  

  

  

  

  

  

  < 
  

  

  /5c 
  M. 
  

  

  13 
  CM. 
  

  

  ^- 
  

  

  to 
  

  

  z 
  

  

  ^ 
  150 
  

  

  z 
  

  

  o 
  

  

  ^ 
  125 
  

  

  Ul 
  

  

  Q 
  

  

  % 
  100 
  

  

  Ul 
  

  

  n 
  

  

  t 
  75 
  

  

  -J 
  

  

  < 
  

  

  

  

  . 
  

  

  

  

  ^^10 
  < 
  

  

  M. 
  

  

  o 
  

  

  > 
  

   o 
  

  

  t 
  

  

  

  

  

  / 
  

  

  

  f 
  

  

  

  

  

  / 
  

  

  V 
  

  

  / 
  

  

  

  

  R 
  

  

  Ul 
  

  

  

  

  i^ 
  

  

  ^ 
  

  

  

  

  

  < 
  

  

  

  / 
  

  

  y 
  

  

  

  -^ 
  

  

  ,^ 
  

  

  ^.^.--''^OM 
  

  

  2 
  ''" 
  

  

  / 
  

  

  7 
  

  

  ^ 
  

  

  

  

  

  

  

  j^ 
  

  

  / 
  / 
  

  

  

  

  

  

  

  

  400 
  200 
  O 
  200 
  400 
  600 
  800 
  1000 
  1200 
  Ohms 
  

   ToTAi 
  Resistance 
  or 
  Instrument 
  Cit?cuit. 
  

  

  Scale 
  Distance 
  = 
  fi9 
  cms. 
  in 
  this 
  Case 
  

  

  this 
  back 
  E.M.F. 
  as 
  low 
  as 
  possible, 
  and 
  how 
  little 
  can 
  

   be 
  gained 
  by 
  reducing 
  the 
  real 
  resistance 
  o£ 
  the 
  wires 
  

   themselves. 
  

  

  The 
  fact 
  that 
  the 
  back 
  E.M.F. 
  of 
  the 
  instrument 
  is 
  practi- 
  

   cally 
  proportional 
  to 
  the 
  deflexion 
  and 
  hence 
  to 
  the 
  current 
  

  

  