﻿796 
  Mr. 
  A. 
  Campbell 
  on 
  Measurement 
  of 
  Wave 
  Length 
  

  

  § 
  4. 
  Measurement 
  of 
  the 
  Self 
  Inductances 
  of 
  the 
  Coils. 
  

  

  The 
  self 
  inductances 
  of 
  all 
  the 
  coils 
  were 
  measured 
  at 
  

   ordinary 
  frequencies 
  (0 
  to 
  1000 
  -^ 
  per 
  sec.) 
  by 
  a 
  method 
  

   specially 
  designed 
  for 
  the 
  accurate 
  measurement 
  of 
  such 
  low 
  

   values 
  (A. 
  Campbell, 
  Phil. 
  Mag. 
  Jan. 
  1908). 
  The 
  com- 
  

   parisons 
  were 
  made 
  against 
  a 
  standard 
  variable 
  mutual 
  

   inductance 
  with 
  ranges 
  of 
  to 
  200 
  and 
  to 
  2000 
  micro- 
  

   henries, 
  the 
  lower 
  range 
  being 
  readable 
  to 
  0*01 
  microhenry. 
  

   The 
  absolute 
  value 
  of 
  this 
  was 
  measured 
  in 
  terms 
  of 
  the 
  new 
  

   Laboratory 
  Standard 
  of 
  mutual 
  inductance, 
  whose 
  value 
  has 
  

   been 
  calculated 
  to 
  a 
  very 
  high 
  degree 
  of 
  accuracy 
  (see 
  Proc. 
  

   Roy. 
  Soc. 
  A. 
  vol. 
  79, 
  1907). 
  The 
  subdivision 
  of 
  the 
  scale 
  of 
  

   the 
  variable 
  mutual 
  inductance 
  was 
  effected 
  by 
  the 
  help 
  of 
  

   Maxwell's 
  method 
  of 
  comparing 
  two 
  mutual 
  inductances, 
  

   Its 
  accuracy 
  was 
  verified 
  by 
  an 
  independent 
  method 
  as 
  

   follows. 
  A 
  mutual 
  inductance 
  coil 
  was 
  constructed 
  with 
  the 
  

   primary 
  and 
  secondary 
  circuits 
  both 
  of 
  well 
  stranded 
  wire 
  

   (10 
  and 
  20 
  strands 
  respectively), 
  and, 
  with 
  all 
  the 
  strands 
  in 
  

   each 
  circuit 
  in 
  series, 
  the 
  value 
  was 
  adjusted 
  to 
  be 
  equal 
  

   to 
  the 
  10 
  microhenry 
  reading 
  on 
  the 
  scale. 
  Then, 
  by 
  taking 
  

   p 
  strands 
  of 
  the 
  primary 
  and 
  q 
  strands 
  of 
  the 
  secondary, 
  the 
  

  

  value 
  would 
  be 
  ^-^ 
  X 
  10, 
  and 
  thus 
  inductances 
  of 
  -g-^Q, 
  -joo^ 
  

  

  2§o, 
  •••of 
  the 
  full 
  scale 
  reading 
  were 
  obtained. 
  On 
  testing 
  

   these 
  against 
  the 
  actual 
  scale, 
  the 
  subdivision 
  was 
  found 
  to 
  

   be 
  perfectly 
  satisfactory. 
  

  

  The 
  details 
  of 
  winding 
  and 
  values 
  of 
  the 
  self 
  inductances 
  

   of 
  the 
  coils 
  of 
  Wavemeter 
  (B) 
  are 
  given 
  in 
  Table 
  I. 
  In 
  

   actual 
  working 
  these 
  values 
  had 
  to 
  be 
  increased 
  by 
  the 
  

   addition 
  of 
  the 
  measured 
  self 
  inductance 
  of 
  the 
  rest 
  of 
  the 
  

   circuit, 
  consisting 
  of 
  the 
  leads, 
  ammeter, 
  and 
  condenser 
  ; 
  the 
  

   total 
  addition 
  was 
  about 
  0*5 
  microhenry, 
  of 
  which 
  0*1 
  micro- 
  

   henry 
  was 
  due 
  to 
  the 
  condenser. 
  

  

  Table 
  I. 
  

  

  Coil 
  

   Coil. 
  Diameter. 
  

  

  j 
  cm. 
  

  

  i 
  

  

  Axial 
  

  

  Length. 
  

  

  cm. 
  

  

  Turns. 
  

  

  Self 
  

   Inductance. 
  

   Microhenries. 
  

  

  QB 
  

  

  Ql 
  

  

  13-5 
  

   7-4 
  

   7-6 
  

  

  3-6 
  

   3-9 
  

   1-7 
  

  

  40 
  

  

  48 
  

   23 
  

  

  360-3 
  

   170-7 
  

  

  57-8, 
  

  

  Q2 
  

  

  

  