﻿for 
  High 
  Frequency 
  Electrical 
  Oscillations, 
  797 
  

  

  § 
  5. 
  Tests 
  of 
  Standard 
  Wavemeter 
  (B) 
  hi/ 
  Pliotograplnng 
  

   Sparks, 
  

  

  In 
  order 
  to 
  obtain 
  an 
  absolute 
  calibration 
  of 
  the 
  standard 
  

   wavemeter, 
  the 
  frequencies 
  of 
  the 
  oscillations 
  with 
  which 
  it 
  

   was 
  tested 
  were 
  determined 
  by 
  including 
  a 
  spark 
  gap 
  in 
  the 
  

   main 
  oscillation 
  circuit 
  to 
  which 
  the 
  wavemeter 
  was 
  loosely 
  

   coupled 
  and 
  photographing 
  the 
  spark 
  trains 
  by 
  help 
  of 
  a 
  

   rotating 
  mirror. 
  

  

  The 
  apparatus 
  was 
  arranged 
  as 
  follows 
  : 
  — 
  

  

  The 
  source 
  of 
  current 
  was 
  a 
  small 
  alternator 
  whose 
  fre- 
  

   quency 
  could 
  be 
  varied 
  from 
  50 
  to 
  200 
  -^ 
  per 
  second 
  ; 
  this 
  

   was 
  connected 
  through 
  a 
  small 
  high 
  voltage 
  transformer 
  to 
  a 
  

   spark 
  gap 
  with 
  cadmium 
  electrodes 
  shunted 
  by 
  a 
  glass 
  plate 
  

   condenser 
  and 
  a 
  bare 
  wire 
  inductance 
  in 
  oil. 
  A 
  large 
  

   variable 
  air 
  condenser 
  consisting 
  of 
  six 
  aluminium 
  disks, 
  each 
  

   100 
  cms. 
  in 
  diameter, 
  could 
  be 
  put 
  in 
  parallel 
  w4th 
  the 
  glass 
  

   plate 
  condenser, 
  and, 
  by 
  suitable 
  variation 
  of 
  the 
  capacity 
  

   and 
  inductance 
  in 
  circuit, 
  oscillations 
  of 
  frequencies 
  from 
  

   300,000 
  to 
  1,200,000 
  — 
  per 
  sec. 
  could 
  be 
  obtained. 
  A 
  

   special 
  camera 
  (fig. 
  2) 
  was 
  constructed 
  and 
  mounted 
  for 
  

   rioriditv 
  on 
  a 
  lono- 
  slab 
  of 
  sandstone. 
  At 
  one 
  end 
  of 
  the 
  

   camei'a 
  was 
  the 
  rotating 
  mirror, 
  which 
  was 
  concave 
  and 
  of 
  

   200 
  cms. 
  radius 
  of 
  curvature. 
  It 
  turned 
  on 
  a 
  vertical 
  axis 
  

   and 
  was 
  driven 
  by 
  a 
  small 
  motor, 
  the 
  speed 
  being 
  kept 
  

   constant 
  by 
  an 
  arrangement 
  described 
  below. 
  The 
  other 
  end 
  

   of 
  the 
  camera 
  had 
  two 
  branches 
  which 
  carried 
  respectively 
  

   the 
  spark 
  electrodes 
  and 
  a 
  pair 
  of 
  guides 
  allowing 
  the 
  plate 
  

   carrier 
  to 
  be 
  smoothly 
  raised 
  or 
  lowered 
  while 
  the 
  rotating 
  

   mirror 
  threw 
  the 
  images 
  of 
  the 
  spark 
  trains 
  as 
  streaks 
  upon 
  

   the 
  plate. 
  The 
  distance 
  from 
  the 
  mirror 
  to 
  the 
  plate 
  was 
  

   200 
  cms. 
  The 
  speed 
  of 
  rotation 
  of 
  the 
  mirror 
  was 
  kept 
  

   constant 
  (usually 
  at 
  about 
  60 
  revs, 
  per 
  second) 
  as 
  follows. 
  

   On 
  the 
  axis 
  was 
  mounted 
  a 
  commutator 
  and 
  this 
  was 
  connected 
  

   with 
  a 
  condenser, 
  a 
  bridge, 
  battery, 
  and 
  galvanometer, 
  for 
  

   Maxwell's 
  Commutator 
  method 
  of 
  measuring 
  capacity. 
  By 
  

   applying 
  a 
  slight 
  variable 
  brake 
  to 
  the 
  rotating 
  axis, 
  the 
  

   galvanometer 
  light-spot 
  could 
  be 
  kept 
  at 
  zero. 
  When 
  this 
  

   was 
  the 
  case 
  the 
  speed 
  was 
  steady, 
  and 
  could 
  be 
  measured 
  

   with 
  a 
  counter 
  or 
  deduced 
  from 
  the 
  value 
  of 
  the 
  condenser 
  

   and 
  the 
  resistance 
  in 
  the 
  bridge. 
  From 
  measurements 
  of 
  a 
  

   number 
  of 
  the 
  spark 
  train 
  photographs 
  on 
  each 
  plate, 
  the 
  

   average 
  displacement 
  from 
  spark 
  to 
  spark 
  was 
  found 
  ; 
  and 
  

   the 
  frequency 
  was 
  calculated 
  from 
  this 
  displacement, 
  the 
  

   distance 
  from 
  the 
  mirror 
  to 
  the 
  plate, 
  and 
  the 
  speed 
  of 
  

   rotation 
  of 
  the 
  mirror. 
  

  

  