﻿800 
  Mr. 
  A. 
  Campbell 
  on 
  Measurement 
  of 
  Wave 
  Length 
  

  

  assuming 
  that 
  the 
  current 
  in 
  that 
  case 
  is 
  practically 
  confined 
  

   to 
  the 
  skin 
  o£ 
  the 
  wire). 
  These 
  values 
  have 
  been 
  calculated 
  

   by 
  the 
  approximate 
  formula 
  (due 
  to 
  Heaviside) 
  :* 
  

  

  T 
  ^ 
  . 
  26^^ad. 
  . 
  1 
  . 
  

   La— 
  L~.= 
  .^^^ 
  , 
  m 
  microhenries, 
  

   " 
  * 
  • 
  iOOO 
  / 
  

  

  where 
  N 
  = 
  number 
  o£ 
  turns, 
  

   a 
  = 
  radius 
  o£ 
  coil, 
  

   I 
  = 
  axial 
  length 
  o£ 
  coil, 
  

   d 
  = 
  diameter 
  of 
  wire, 
  

   all 
  the 
  dimensions 
  being 
  in 
  centimetres. 
  

  

  The 
  appHcation 
  of 
  the 
  formula 
  to 
  such 
  short 
  coils 
  is 
  not 
  

   quite 
  appropriate, 
  however, 
  as 
  it 
  has 
  been 
  deduced 
  on 
  the 
  

   assumption 
  that 
  the 
  solenoid 
  is 
  long. 
  

  

  The 
  two 
  wavemeters 
  were 
  then 
  loosely 
  coupled 
  to 
  the 
  same 
  

   oscillation 
  circuit 
  and 
  simultaneous 
  readings 
  taken 
  at 
  various 
  

   frequencies. 
  From 
  the 
  calibration 
  of 
  (B) 
  already 
  established 
  

   and 
  the 
  results 
  in 
  Table 
  III. 
  the 
  effective 
  values 
  of 
  the 
  self 
  

   inductances 
  of 
  the 
  coils 
  of 
  (A) 
  were 
  deduced 
  ; 
  they 
  are 
  given 
  

   in 
  Table 
  Y. 
  

  

  Table 
  V. 
  

  

  Coil. 
  

  

  Comparison 
  

   Frequency 
  

   — 
  . 
  per 
  sec. 
  

  

  Effective 
  L 
  

  

  (deduced). 
  

  

  Microhenries. 
  

  

  L. 
  

  

  (Mean). 
  

  

  A4 
  

  

  1,125,000 
  

   1,338,000 
  

  

  837,400 
  

   975,500 
  

  

  665,100 
  

  

  366,700 
  

   394,800 
  

   466,800 
  

   473,400 
  

   665,100 
  

  

  284,500 
  

   290,300 
  

   322,800 
  

  

  290,300 
  

  

  20-1 
  

   19-8 
  

  

  42-6 
  

   42-5 
  

  

  991 
  

  

  190-5 
  

   190-5 
  

   191-1 
  

   192-0 
  

   191-8 
  

  

  530 
  

   530 
  

   532-5 
  

  

  104. 
  

  

  19-9 
  

  

  42-5,3 
  

   99-1 
  

  

  191-2 
  

  

  53, 
  

   104^ 
  

  

  A5 
  

  

  A6 
  

  

  A7 
  

  

  AS 
  

  

  A9 
  

  

  

  

  

  Collected 
  Papers, 
  vol. 
  i., 
  p. 
  356. 
  

  

  