﻿342 
  Mr. 
  W. 
  Duddell 
  on 
  

  

  were 
  kept 
  constant, 
  the 
  resistance 
  being 
  137 
  ohms 
  and 
  the 
  

   frequency 
  75 
  interruptions 
  per 
  second. 
  The 
  current 
  through 
  

   the 
  primary 
  of 
  the 
  coil 
  was 
  about 
  1/4 
  ampere. 
  

  

  A 
  large 
  number 
  of 
  points 
  were 
  taken 
  and 
  are 
  plotted 
  on 
  

   the 
  curve. 
  They 
  do 
  not 
  give 
  very 
  consistent 
  results, 
  so 
  it 
  is 
  

   difficult 
  to 
  draw 
  a 
  smooth 
  curve 
  through 
  them 
  ; 
  but 
  the 
  

   general 
  appearance 
  of 
  the 
  phenomenon 
  can 
  be 
  seen. 
  On 
  

   introducing 
  a 
  spark-gap 
  1/10 
  mm 
  long 
  the 
  root-mean-squared 
  

   current 
  instantly 
  rose 
  from 
  3'8 
  to 
  38*5 
  milliamperes, 
  and 
  

   continued 
  to 
  increase 
  with 
  increasing 
  spark-length 
  until 
  it 
  

   reached 
  a 
  maximum 
  at 
  a 
  spark-length 
  somewhere 
  in 
  the 
  

   neighbourhood 
  of 
  1"1 
  mm. 
  The 
  exact 
  point 
  is 
  slightly 
  un- 
  

   certain 
  owing 
  to 
  the 
  R.M.S. 
  current 
  just 
  exceeding 
  the 
  

   range 
  of 
  the 
  instrument 
  that 
  was 
  in 
  use. 
  From 
  1*6 
  to 
  5 
  mm. 
  

   spark-length 
  the 
  R.M.S. 
  current 
  gradually 
  fell 
  in 
  value 
  and 
  

   attained 
  a 
  minimum 
  value 
  of 
  10-J- 
  milliamperes. 
  Further 
  

   increase 
  in 
  spark-length 
  produced 
  a 
  gradual 
  increase 
  in 
  the 
  

   R.M.S. 
  current 
  up 
  to 
  the 
  maximum 
  length 
  of 
  15 
  mm. 
  that 
  

   was 
  used 
  in 
  the 
  experiments. 
  

  

  There 
  is 
  no 
  doubt 
  in 
  my 
  mind 
  as 
  to 
  the 
  cause 
  of 
  this 
  effect. 
  

   It 
  is 
  due 
  to 
  very 
  high 
  frequency 
  oscillations 
  being 
  set 
  up 
  in 
  

   the 
  wires 
  connected 
  to 
  the 
  secondary 
  circuit 
  of 
  the 
  coil 
  when 
  

   a 
  spark-gap 
  is 
  introduced. 
  The 
  magnitude 
  of 
  the 
  oscillations 
  

   will 
  depend 
  on 
  the 
  voltage 
  between 
  the 
  terminals 
  of 
  the 
  spark- 
  

   gap 
  just 
  before 
  the 
  spark 
  passes 
  and 
  on 
  the 
  resistance 
  that 
  

   the 
  spark-gap 
  offers. 
  Now 
  the 
  P.D. 
  between 
  the 
  terminals 
  

   of 
  the 
  spark-gap 
  will 
  increase 
  with 
  increasing 
  length 
  and 
  so 
  

   will 
  the 
  resistance, 
  so 
  that 
  on 
  increasing 
  the 
  spark-length 
  we 
  

   have 
  two 
  conflicting 
  agencies 
  at 
  work, 
  one 
  tending 
  to 
  increase 
  

   the 
  magnitude 
  of 
  the 
  oscillatory 
  current, 
  and 
  the 
  second 
  

   tending 
  to 
  decrease 
  the 
  magnitude. 
  I 
  think 
  that 
  it 
  is 
  due 
  to 
  

   this 
  differential 
  action 
  that 
  the 
  curve 
  is 
  such 
  a 
  curious 
  shape. 
  

  

  The 
  presence 
  of 
  the 
  oscillations 
  in 
  the 
  secondary 
  circuit 
  can 
  

   easily 
  be 
  made 
  evident 
  by 
  taking 
  a 
  well 
  insulated 
  metal 
  plate 
  

   and 
  touching 
  various 
  points 
  in 
  the 
  secondary 
  circuit 
  with 
  it. 
  

   The 
  effect 
  of 
  this 
  plate 
  will 
  be 
  to 
  largely 
  increase 
  or 
  decrease, 
  

   generally 
  increase, 
  the 
  value 
  of 
  the 
  R.M.S. 
  current. 
  The 
  

   practical 
  aspect 
  of 
  this 
  question 
  from 
  my 
  point 
  of 
  view 
  was 
  

   that, 
  owing 
  to 
  the 
  unexpectedly 
  large 
  value 
  of 
  the 
  R.M.S. 
  

   current, 
  I 
  burnt 
  up 
  several 
  thermo-ammeters 
  before 
  I 
  dis- 
  

   covered 
  the 
  cause 
  of 
  the 
  trouble. 
  

  

  I 
  have 
  obtained 
  the 
  effect 
  with 
  brass, 
  iron, 
  zinc, 
  and 
  

   aluminium 
  electrodes 
  and 
  it 
  probably 
  takes 
  place 
  with 
  all 
  

   other 
  metals. 
  I 
  think 
  that, 
  so 
  far, 
  the 
  best 
  metal 
  to 
  show 
  the 
  

   effect 
  has 
  been 
  aluminium. 
  

  

  