﻿Short 
  Spark 
  Phenomena. 
  

  

  341 
  

  

  When 
  there 
  was 
  no 
  spark-gap 
  in 
  the 
  secondary 
  circuit 
  and 
  

   the 
  coil 
  was 
  in 
  action, 
  the 
  mean 
  current, 
  as 
  read 
  by 
  the 
  

   galvanometer, 
  was 
  zero, 
  as 
  it 
  should 
  be, 
  and 
  the 
  root-mean- 
  

   squared 
  current 
  had 
  a 
  value 
  of 
  about 
  3*8 
  milliamperes. 
  

  

  I£ 
  now 
  a 
  microscopic 
  spark-gap, 
  say 
  between 
  two 
  aluminium 
  

   points, 
  is 
  introduced 
  into 
  the 
  secondary 
  circuit 
  two 
  curious 
  

   effects 
  take 
  place. 
  Firstly, 
  the 
  R.M.S. 
  current 
  enormously 
  

   increases 
  in 
  value 
  ; 
  and 
  secondly, 
  a 
  very 
  large 
  deflexion 
  is 
  

   produced 
  on 
  the 
  galvanometer, 
  reading 
  the 
  mean 
  current, 
  

   and 
  this 
  deflexion 
  is 
  in 
  the 
  negative 
  direction, 
  that 
  is 
  to 
  say, 
  

   in 
  the 
  direction 
  corresponding 
  to 
  making 
  the 
  primary 
  circuit. 
  

   I 
  will 
  consider 
  these 
  two 
  effects 
  separately. 
  

  

  In 
  order 
  to 
  give 
  an 
  idea 
  of 
  the 
  magnitude 
  of 
  the 
  increase 
  

   in 
  the 
  R.M.S. 
  current 
  produced 
  by 
  introducing 
  a 
  very 
  small 
  

   spark-gap 
  into 
  the 
  secondary 
  circuit 
  of 
  the 
  coil, 
  I 
  have 
  plotted 
  

   in 
  fig. 
  2 
  the 
  R.M.S. 
  current 
  corresponding 
  to 
  various 
  

  

  Fig. 
  2. 
  

  

  70 
  

  

  60 
  

  

  SO 
  

  

  

  

  

  

  

  

  

  

  t'i 
  

  

  

  

  

  

  

  

  

  \ 
  

  

  

  

  

  

  

  

  

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  \ 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  \ 
  

  

  V- 
  

  

  

  

  

  -""■"" 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  * 
  40 
  ' 
  

  

  §30 
  

   ^ 
  20\- 
  

  

  10 
  

  

  4 
  6 
  8/0 
  12 
  

  

  Iea'gth 
  of 
  Spark 
  G/?p//v 
  M/lumtirs 
  

  

  /4 
  

  

  !6 
  

  

  lengths 
  of 
  spark 
  between 
  and 
  15 
  mm. 
  In 
  making 
  these 
  

   observations 
  the 
  resistance 
  in 
  the 
  primary 
  circuit 
  of 
  the 
  coil 
  

   and 
  the 
  frequency 
  of 
  the 
  interrupter 
  of 
  the 
  primary 
  current 
  

  

  