﻿High 
  Frequency 
  Electrical 
  Oscillations. 
  795 
  

  

  The 
  combination 
  is 
  capable 
  of 
  measuring 
  oscillation 
  fre- 
  

   quencies 
  over 
  a 
  range 
  extending 
  from 
  about 
  20,000,000 
  down 
  

   to 
  100,000 
  ^' 
  per 
  sec, 
  or 
  wave-lengths 
  from 
  X=15 
  metres 
  up 
  

   to 
  X 
  = 
  3000 
  metres. 
  It 
  must 
  be 
  kept 
  in 
  mind, 
  however, 
  that 
  

   the 
  accuracy 
  obtainable 
  depends 
  on 
  the 
  part 
  of 
  the 
  condenser 
  

   scale 
  at 
  which 
  the 
  reading 
  is 
  taken. 
  For 
  example, 
  at 
  a 
  

   reading 
  of 
  20 
  the 
  frequency 
  can 
  barely 
  be 
  read 
  to 
  0*5 
  per 
  

   cent. 
  

  

  As 
  the 
  coils 
  (A 
  1, 
  A 
  2, 
  ) 
  of 
  (A) 
  are 
  of 
  solid 
  (not 
  

  

  stranded) 
  wire 
  of 
  diameters 
  from 
  0*32 
  up 
  to 
  3*2 
  mm., 
  the 
  

   values 
  of 
  their 
  self 
  inductances 
  at 
  the 
  high 
  frequencies 
  are, 
  

   for 
  most 
  of 
  the 
  coils, 
  considerably 
  lower 
  than 
  those 
  obtained 
  

   at 
  ordinary 
  frequencies 
  of 
  to 
  1000 
  -^ 
  per 
  sec. 
  

  

  With 
  a 
  view 
  to 
  checking 
  the 
  results 
  of 
  the 
  direct 
  experi- 
  

   ments 
  by 
  calculation 
  from 
  the 
  measured 
  inductance 
  and 
  

   capacity 
  of 
  a 
  wavemeter 
  in 
  which 
  the 
  inductances 
  would 
  be 
  

   much 
  less 
  affected 
  by 
  frequency, 
  and 
  thus 
  to 
  obtain 
  a 
  standard 
  

   instrument 
  for 
  future 
  use, 
  we 
  constructed 
  a 
  wavemeter 
  (B) 
  

   in 
  which 
  the 
  coils 
  are 
  all 
  of 
  highly 
  stranded 
  wire. 
  

  

  § 
  3. 
  Description 
  of 
  Standard 
  Wavemeter 
  (B). 
  

  

  The 
  general 
  arrangement 
  of 
  (B) 
  was 
  similar 
  to 
  that 
  of 
  (A). 
  

   The 
  variable 
  condenser 
  (from 
  100 
  to 
  900 
  mmfd.) 
  was 
  of 
  

   special 
  design, 
  wdth 
  amberite 
  washers 
  to 
  give 
  very 
  high 
  

   insulation. 
  It 
  was 
  kindly 
  presented 
  to 
  the 
  Laboratory 
  by 
  

   Dr. 
  Alexander 
  Muirhead, 
  F.R.S. 
  We 
  added 
  to 
  it 
  a 
  direct 
  

   reading 
  scale, 
  which 
  can 
  be 
  read 
  to 
  1 
  in 
  2000 
  at 
  the 
  upper 
  

   end 
  and 
  to 
  1 
  in 
  200 
  at 
  the 
  lower. 
  The 
  scale 
  was 
  constructed 
  

   by 
  a 
  very 
  careful 
  series 
  of 
  tests 
  by 
  Maxwell's 
  Commutator 
  

   Method 
  and 
  was 
  found 
  to 
  be 
  very 
  uniform. 
  

  

  For 
  the 
  inductances, 
  three 
  coils 
  (Q 
  Z>, 
  Q 
  1, 
  and 
  Q 
  2) 
  were 
  

   used. 
  They 
  w^ere 
  all 
  w-ound 
  on 
  ebonite 
  tubes 
  with 
  stranded 
  

   wire 
  (7/36*), 
  i. 
  e. 
  containing 
  seven 
  insulated 
  strands, 
  each 
  of 
  

   diameter 
  0*19 
  mm. 
  In 
  each 
  coil 
  the 
  terminals 
  were 
  brought 
  

   to 
  a 
  considerable 
  distance 
  (18*5 
  cm.) 
  from 
  the 
  centre 
  of 
  the 
  

   coil 
  by 
  fixed 
  leads 
  run 
  parallel 
  to 
  one 
  another 
  1 
  cm. 
  apart. 
  

   In 
  this 
  way 
  too 
  close 
  proximity 
  between 
  the 
  coil 
  and 
  the 
  

   condenser 
  plates 
  was 
  avoided. 
  A 
  Duddell 
  thermo-ammeter 
  of 
  

   1*5 
  ohms 
  resistance 
  was 
  used 
  to 
  complete 
  the 
  circuit 
  of 
  

   the 
  coil 
  and 
  the 
  condenser, 
  and 
  by 
  observing 
  its 
  maximum 
  

   deflexion 
  the 
  point 
  of 
  resonance 
  was 
  obtained. 
  The 
  sensi- 
  

   tivity 
  of 
  course 
  varies 
  wath 
  the 
  coil 
  used 
  and 
  with 
  the 
  nature 
  

   of 
  the 
  oscillatory 
  circuit 
  ; 
  it 
  was 
  found 
  to 
  be 
  sufficient 
  for 
  

   the 
  purpose 
  in 
  the 
  experiments 
  described 
  below. 
  Fig. 
  1 
  

   (PI. 
  XXVIII.) 
  shows 
  a 
  photograph 
  of 
  the 
  wavemeter 
  and 
  of 
  

   one 
  of 
  the 
  coils 
  separately. 
  

  

  3 
  G 
  2 
  

  

  