﻿a" 
  \ 
  . 
  

  

  .^ 
  "2 
  is 
  neglected. 
  

  

  428 
  Dr. 
  J. 
  W. 
  Nicholson 
  on 
  Inductance 
  and 
  

  

  and 
  thus 
  for 
  iron 
  wires 
  with 
  \ 
  > 
  8^2, 
  

  

  where 
  

  

  A, 
  = 
  47ra 
  r— 
  1 
  , 
  and 
  

  

  On 
  account 
  of 
  yu-, 
  this 
  formula 
  may 
  be 
  applied, 
  for 
  any- 
  

   given 
  frequency, 
  to 
  a 
  wire 
  of 
  only 
  about 
  a 
  quarter 
  the 
  corre- 
  

   sponding 
  limiting 
  radius 
  in 
  the 
  case 
  of 
  copper. 
  For 
  example, 
  

   for 
  a 
  frequency 
  800, 
  and 
  radius 
  2 
  millimetres, 
  the 
  value 
  of 
  x 
  

   becomes 
  5, 
  with 
  //,= 
  100. 
  Thus 
  a 
  three-figure 
  accuracy 
  is 
  

   obtained 
  for 
  the 
  most 
  unfavourable 
  case 
  of 
  the 
  telephone 
  

   circuit. 
  Accordingly, 
  for 
  a 
  telephone 
  circuit 
  of 
  iron 
  wires 
  

   not 
  twisted 
  together, 
  this 
  result 
  is 
  of 
  universal 
  application. 
  

  

  Iron 
  Wires 
  with 
  Small 
  Frequency. 
  

  

  This 
  case 
  is 
  not 
  very 
  important, 
  but 
  the 
  result 
  may 
  be 
  

   given 
  at 
  once. 
  Thus 
  if 
  x 
  is 
  not 
  greater 
  than 
  2, 
  we 
  may 
  

   neglect 
  the 
  term 
  involving 
  //,"2, 
  and 
  write, 
  to 
  three 
  significant 
  

   figures, 
  

  

  The 
  values 
  of 
  (a, 
  ^, 
  7) 
  suited 
  to 
  this 
  case 
  have 
  been 
  given 
  

   by 
  Russell 
  *. 
  

  

  If 
  X 
  /2nAi 
  

  

  42 
  . 
  7 
  ! 
  ^ 
  J 
  

  

  11 
  , 
  473 
  

   24^ 
  "^3.6! 
  

  

  304107 
  

  

  4.122 
  

  

  

  12 
  720 
  

   /S 
  = 
  ^.^(l- 
  

  

  7 
  = 
  ^(1-^^* 
  + 
  

  

  Moreover, 
  

  

  ^ 
  - 
  1 
  ^ 
  A 
  -1.4 
  . 
  J3_ 
  647 
  ^,,\ 
  

  

  a"2"\ 
  24'^ 
  "^4320" 
  122.360.56''/ 
  

  

  19 
  

   5! 
  

  

  687 
  

  

  7.6 
  

  

  '-■"•) 
  

  

  (39) 
  

  

  

  12 
  

  

  180 
  

   * 
  Loc. 
  cit. 
  

  

  12.28.30 
  

  

  ■■) 
  

  

  (40) 
  

  

  