﻿Short 
  Spark 
  Phenomena. 
  343 
  

  

  The 
  large 
  deflexion 
  in 
  the 
  negative 
  direction 
  observed 
  on 
  

   the 
  galvanometer 
  was 
  investigated 
  by 
  recording 
  the 
  wave- 
  

   forms 
  of 
  the 
  P. 
  D. 
  and 
  the 
  current 
  by 
  means 
  of 
  an 
  oscillograph. 
  

   The 
  sensibilitv 
  of 
  the 
  oscillograph 
  was 
  adjusted 
  so 
  that 
  1mm. 
  

   deflexion 
  equals 
  1 
  milliampere 
  for 
  the 
  current 
  wave-forms. 
  

   To 
  obtain 
  the 
  P.D. 
  wave-forms 
  a 
  water 
  resistance 
  of 
  about 
  

   1 
  meoohm 
  was 
  placed 
  in 
  series 
  with 
  the 
  second 
  moving 
  system 
  

   of 
  the 
  oscillograph 
  so 
  that 
  1 
  mm. 
  equals 
  about 
  1000 
  volts. 
  

   The 
  speed 
  of 
  the 
  plate 
  on 
  which 
  the 
  records 
  were 
  taken 
  was 
  

   1500 
  mm. 
  per 
  second. 
  For 
  this 
  series 
  of 
  tests, 
  the 
  frequency 
  

   of 
  interruption 
  was 
  75 
  per 
  second 
  and 
  the 
  resistance 
  in 
  series 
  

   with 
  primary 
  of 
  the 
  induction-coil 
  was 
  37 
  ohms. 
  

  

  Records 
  were 
  made 
  for 
  a 
  series 
  of 
  spark-lengths 
  between 
  

   aluminium-point 
  electrodes. 
  I 
  have 
  selected 
  from 
  these 
  some 
  

   typical 
  results 
  which 
  are 
  shown 
  in 
  Plates 
  XIII.-XV. 
  figs. 
  3, 
  

   4, 
  5, 
  6, 
  7, 
  8. 
  

  

  Fio\ 
  3 
  is 
  the 
  current 
  wave-form 
  when 
  the 
  spark-gap 
  is 
  

   short-circuited, 
  length 
  0. 
  The 
  straight 
  line 
  across 
  the 
  centre 
  

   of 
  the 
  figure 
  is 
  the 
  true 
  zero 
  line. 
  Deflex 
  ! 
  ons 
  above 
  this 
  

   zero 
  line 
  represent 
  current 
  in 
  the 
  positive 
  direction 
  and 
  below 
  

   the 
  zero 
  line 
  represent 
  current 
  in 
  the 
  negative 
  direction. 
  It 
  

   will 
  be 
  noticed 
  that 
  the 
  maximum 
  current 
  in 
  the 
  positive 
  

   direction 
  (14*5 
  milliamperes) 
  is 
  much 
  less 
  thau 
  the 
  maximum 
  

   current 
  in 
  the 
  negative 
  direction 
  (35 
  milliamperes), 
  but 
  the 
  

   length 
  of 
  time 
  that 
  the 
  negative 
  current 
  lasts 
  is 
  much 
  shorter 
  

   than 
  that 
  which 
  the 
  positive 
  current 
  lasts 
  ; 
  so 
  that 
  the 
  areas 
  

   of 
  the 
  two 
  sides 
  of 
  the 
  zero 
  line 
  are 
  equal 
  and 
  the 
  mean 
  

   current 
  zero. 
  

  

  The 
  smallest 
  gap 
  that 
  I 
  could 
  make 
  in 
  this 
  circuit 
  between 
  

   the 
  aluminium 
  electrodes, 
  at 
  once 
  changed 
  the 
  wave-form 
  to 
  

   the 
  type 
  shown 
  in 
  fig. 
  4, 
  which 
  is 
  for 
  spark-length 
  1 
  mm. 
  

   In 
  this 
  figure, 
  the 
  straight 
  parts 
  of 
  the 
  curve 
  along 
  the 
  centre 
  

   of 
  the 
  figure 
  are 
  in 
  the 
  position 
  of 
  zero 
  current. 
  This 
  was 
  

   carefully 
  checked 
  by 
  taking 
  records 
  with 
  a 
  fixed 
  datum-line 
  

   at 
  the 
  zero 
  of 
  the 
  curve. 
  The 
  line 
  was 
  afterwards 
  moved 
  to 
  

   the 
  lower 
  part 
  of 
  the 
  figure 
  in 
  order 
  not 
  to 
  hide 
  small 
  details 
  

   of 
  the 
  curve 
  near 
  the 
  zero. 
  The 
  effect 
  of 
  introducing 
  the 
  

   spark-gap 
  of 
  1 
  mm. 
  is, 
  w 
  T 
  hile 
  leaving 
  the 
  maximum 
  current 
  

   on 
  the 
  two 
  sides 
  of 
  the 
  zero 
  at 
  practically 
  the 
  same 
  value, 
  to 
  

   reduce 
  the 
  area 
  of 
  the 
  curve 
  on 
  the 
  positive 
  side 
  of 
  the 
  zero 
  

   line 
  nearly 
  to 
  zero, 
  so 
  that 
  instead 
  of 
  the 
  areas 
  on 
  the 
  two 
  sides 
  

   being 
  equal 
  there 
  is 
  a 
  large 
  excess 
  of 
  area 
  on 
  the 
  negative 
  side 
  

   causing 
  a 
  large 
  mean 
  current 
  in 
  the 
  negative 
  direction. 
  

  

  On 
  increasing 
  the 
  length 
  of 
  the 
  gap 
  a 
  small 
  area 
  on 
  the 
  

   opposite 
  side 
  of 
  the 
  zero 
  line 
  again 
  begins 
  to 
  form, 
  which 
  

  

  