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  XXVI. 
  Short 
  Spark 
  Phenomena. 
  

   By 
  W. 
  Duddell, 
  F.R.S* 
  

  

  [Plates 
  XIII.-XV.] 
  

  

  IN 
  connexion 
  with 
  some 
  measurements 
  of 
  the 
  current 
  in 
  

   the 
  secondary 
  circuit 
  o£ 
  an 
  induction-coil, 
  I 
  have 
  noticed 
  

   two 
  curious 
  effects, 
  which 
  are 
  probably 
  well 
  known 
  but 
  

   which 
  I 
  do 
  not 
  remember 
  having 
  seen 
  described 
  anywhere. 
  

  

  The 
  apparatus 
  in 
  use 
  consisted 
  of 
  a 
  12-inch 
  Newton 
  induction- 
  

   coil 
  which 
  was 
  supplied 
  from 
  the 
  200 
  volt 
  direct-current 
  

   mains. 
  A 
  large 
  resistance 
  was 
  placed 
  in 
  series 
  with 
  the 
  

   primary 
  of 
  the 
  coil 
  to 
  limit 
  the 
  current, 
  and 
  the 
  current 
  was 
  

   interrupted 
  by 
  means 
  of 
  a 
  mercury-jet 
  interrupter; 
  the 
  

   connexions 
  are 
  shown 
  in 
  fig. 
  1. 
  The 
  secondary 
  circuit 
  

  

  Fig. 
  1. 
  

  

  Interrupter 
  

  

  contained 
  a 
  galvanometer 
  G 
  to 
  measure 
  the 
  mean 
  current 
  

   and 
  thermo-ammeter 
  T. 
  A. 
  to 
  measure 
  the 
  root-mean-squared 
  

   current. 
  

  

  The 
  galvanometer 
  was 
  specially 
  constructed 
  for 
  the 
  purpose 
  

   so 
  as 
  to 
  have 
  a 
  sufficiently 
  low 
  sensibility 
  without 
  using 
  a 
  

   shunt. 
  It 
  was 
  of 
  the 
  moving-coil 
  type 
  and 
  was 
  well 
  insulated 
  

   from 
  earth 
  by 
  means 
  of 
  porcelain 
  insulators. 
  The 
  sensibility 
  

   was 
  such 
  that 
  1 
  milliampere 
  gave 
  a 
  scale-deflexion 
  of 
  200 
  

   divisions 
  (1 
  division 
  equals 
  1/40 
  in.). 
  The 
  thermo-ammeter 
  

   had 
  a 
  resistance 
  of 
  about 
  101*5 
  ohms 
  and 
  gave 
  its 
  full 
  scale- 
  

   deflexion 
  for 
  about 
  70 
  milliamperes 
  R.M.S. 
  value. 
  

  

  By 
  breaking 
  the 
  current 
  through 
  the 
  primary 
  by 
  means 
  of 
  

   a 
  switch, 
  the 
  direction 
  of 
  the 
  deflexion 
  of 
  the 
  galvanometer 
  

   corresponding 
  to 
  breaking 
  the 
  primary 
  current 
  was 
  

   determined. 
  A 
  deflexion, 
  in 
  this 
  direction 
  I 
  will 
  call, 
  in 
  

   what 
  follows, 
  a 
  positive 
  deflexion, 
  and 
  a 
  deflexion 
  in 
  the 
  

   opposite 
  direction, 
  that 
  is 
  corresponding 
  to 
  the 
  make 
  of 
  the 
  

   primary 
  current, 
  I 
  will 
  call 
  a 
  negative 
  deflexion. 
  

  

  * 
  Communicated 
  by 
  the 
  Physical 
  Society 
  : 
  read 
  April 
  10, 
  1908. 
  

  

  