﻿798 
  Mr. 
  A. 
  Campbell 
  on 
  Measurement 
  of 
  Wave 
  Length 
  

  

  While 
  each 
  photograph 
  was 
  being 
  taken 
  the 
  reading 
  of 
  the 
  

   standard 
  wavemeter 
  vvas 
  also 
  observed, 
  care 
  being 
  taken 
  to 
  

   keep 
  the 
  coupling 
  to 
  the 
  spark 
  circuit 
  very 
  loose. 
  

  

  The 
  value 
  (n) 
  o£ 
  ihe 
  frequency 
  obtained 
  from 
  the 
  spark 
  

   photographs 
  was 
  in 
  each 
  case 
  compared 
  with 
  the 
  values 
  (ni) 
  

   calculated 
  from 
  the 
  measured 
  values 
  K 
  and 
  L 
  of 
  the 
  capacity 
  

   and 
  inductance 
  in 
  the 
  wavemeter 
  circuit. 
  Since 
  the 
  wave- 
  

   meter 
  coils 
  have 
  a 
  certain 
  amount 
  of 
  distributed 
  capacity, 
  it 
  

   was 
  necessary 
  to 
  take 
  account 
  of 
  this. 
  The 
  required 
  correc- 
  

   tion 
  was 
  made 
  by 
  Glazebrook 
  and 
  Lodge's 
  formula 
  (Cambridge 
  

   Phil. 
  Trans, 
  p. 
  171, 
  vol. 
  xviii. 
  1899), 
  

  

  vfheYep 
  = 
  27rn, 
  N 
  = 
  number 
  of 
  turns 
  in 
  coil, 
  /: 
  = 
  capacity 
  from 
  

   turn 
  to 
  turn^ 
  and 
  the 
  capacities 
  and 
  inductances 
  are 
  in 
  

   absolute 
  measure. 
  The 
  value 
  of 
  k 
  was 
  found 
  by 
  testing 
  the 
  

   capacities 
  of 
  coils 
  with 
  bifilar 
  windings 
  of 
  wire 
  similar 
  to 
  that 
  

   in 
  the 
  coils 
  used. 
  The 
  correction 
  is 
  small, 
  being 
  in 
  no 
  case 
  

   more 
  than 
  1 
  part 
  in 
  1000. 
  The 
  results 
  are 
  given 
  in 
  Table 
  II. 
  

  

  Table 
  II. 
  

  

  

  n 
  

  

  r/j 
  

  

  Plates 
  

   Nos. 
  

  

  By 
  Sparks 
  

   -^ 
  per 
  sec. 
  

  

  From 
  K 
  & 
  L 
  of 
  

  

  Standard 
  Wavemeter. 
  

  

  -^^ 
  per 
  sec. 
  

  

  61&62 
  

  

  290,300 
  

  

  290,500 
  

  

  47 
  

  

  516,800 
  

  

  516,800 
  

  

  57&58 
  

  

  818,300 
  

  

  821,200 
  

  

  55 
  

  

  1,042,000 
  

  

  1,039,000 
  

  

  We 
  may 
  remark 
  that 
  the 
  small 
  differences 
  between 
  n 
  and 
  

   ni 
  are 
  quite 
  within 
  the 
  limits 
  of 
  probable 
  experimental 
  error. 
  

   On 
  the 
  same 
  plate 
  the 
  value 
  of 
  n 
  deduced 
  from 
  the 
  various 
  

   spark 
  trains 
  sometimes 
  showed 
  an 
  extreme 
  variation 
  of 
  1'2 
  

   per 
  cent, 
  from 
  the 
  mean 
  ; 
  in 
  the 
  best 
  experiments 
  the 
  

   variation 
  was 
  about 
  0*5 
  per 
  cent, 
  from 
  the 
  mean. 
  In 
  

   general 
  the 
  mean 
  of 
  5 
  to 
  10 
  spark 
  trains 
  was 
  used 
  and 
  the 
  

   average 
  variation 
  from 
  the 
  mean 
  was 
  from 
  0*2 
  to 
  0*6 
  per 
  

   cent, 
  in 
  the 
  value 
  of 
  n. 
  

  

  