﻿344 
  Short 
  Spark 
  Phenomena. 
  

  

  increases 
  with 
  increasing 
  length 
  of 
  gap, 
  until 
  the 
  condition 
  

   shown 
  in 
  fig. 
  6 
  is 
  reached. 
  In 
  this 
  figure, 
  the 
  spark 
  is 
  some- 
  

   times 
  rectifying 
  or 
  stopping 
  the 
  current 
  flowing 
  round 
  the 
  

   circuit 
  in 
  the 
  negative 
  direction 
  altogether. 
  We 
  have 
  in 
  this 
  

   figure 
  the 
  large 
  triangular 
  current 
  wave 
  -form 
  which 
  corre- 
  

   sponds 
  to 
  the 
  current 
  flowing 
  in 
  only 
  one 
  direction 
  round 
  the 
  

   circuit, 
  and 
  the 
  smaller 
  triangular 
  current 
  pulses 
  accompanied 
  

   by 
  a 
  large 
  current 
  in 
  the 
  negative 
  direction 
  at 
  make. 
  

  

  Further 
  lengthening 
  the 
  spark-gap 
  brings 
  it 
  into 
  the 
  

   normal 
  condition 
  of 
  long 
  sparks 
  ; 
  the 
  current 
  wave 
  of 
  this 
  is 
  

   shown 
  in 
  fig. 
  7. 
  The 
  datum-line 
  across 
  the 
  centre 
  of 
  the 
  

   plate, 
  in 
  this 
  case, 
  is 
  true 
  zero 
  line. 
  

  

  It 
  was 
  necessary 
  in 
  order 
  to 
  obtain 
  the 
  wave-form 
  of 
  these 
  

   longer 
  lengths 
  to 
  disconnect 
  the 
  circuit 
  for 
  recording 
  the 
  P.D. 
  

   wave-form, 
  as 
  the 
  leak 
  which 
  it 
  formed, 
  having 
  a 
  resistance 
  

   of 
  only 
  1 
  megohm, 
  prevented 
  (he 
  sparks 
  passing 
  across 
  the 
  

   gap. 
  

  

  Fig. 
  8 
  shows 
  the 
  P.D. 
  wave-form 
  with 
  so 
  long 
  a 
  spark- 
  

   gap 
  that 
  the 
  spark 
  could 
  not 
  jump 
  across 
  it. 
  This 
  is 
  the 
  

   normal 
  wave-form 
  given 
  on 
  the 
  secondary 
  of 
  the 
  induction- 
  

   coil 
  when 
  supplied 
  from 
  a 
  high 
  voltage 
  direct-current 
  

   circuit. 
  

  

  It 
  will 
  be 
  noted 
  that 
  the 
  voltage 
  induced 
  in 
  the 
  circuit 
  at 
  

   the 
  break 
  is 
  about 
  23,000 
  volts 
  and 
  at 
  make 
  about 
  13,500, 
  so 
  

   that 
  the 
  make 
  voltage 
  exceeds 
  one 
  half 
  of 
  the 
  break 
  voltage. 
  

   Also, 
  the 
  voltage 
  induced 
  at 
  the 
  make 
  dies 
  away 
  less 
  rapidly 
  

   than 
  that 
  induced 
  at 
  the 
  break. 
  It 
  may 
  be 
  enquired 
  how 
  it 
  

   is 
  possible, 
  if 
  the 
  make 
  voltage 
  is 
  less 
  than 
  the 
  break 
  voltage, 
  

   for 
  the 
  current 
  to 
  be 
  larger 
  at 
  make 
  than 
  at 
  break. 
  I 
  think 
  

   that 
  this 
  question 
  must 
  be 
  answered 
  by 
  noting 
  that 
  during 
  

   the 
  make 
  period 
  the 
  primary 
  of 
  the 
  induction-coil 
  is 
  connected 
  

   to 
  the 
  supply 
  mains 
  so 
  that 
  the 
  energy 
  may 
  be 
  directly 
  trans- 
  

   ferred 
  from 
  the 
  primary 
  to 
  the 
  secondary 
  circuit 
  by 
  magnetic 
  

   induction 
  ; 
  that 
  is 
  to 
  say, 
  as 
  long 
  as 
  we 
  maintain 
  a 
  steadily 
  

   increasing 
  flow 
  of 
  current 
  into 
  the 
  primary, 
  we 
  can 
  continue 
  

   to 
  take 
  energy 
  from 
  the 
  secondary. 
  During 
  the 
  break 
  period, 
  

   however, 
  things 
  are 
  very 
  different. 
  The 
  whole 
  of 
  the 
  energy 
  

   that 
  we 
  can 
  get 
  out 
  of 
  the 
  secondary 
  is 
  that 
  stored 
  up 
  in 
  the 
  

   magnetic 
  field 
  which 
  is 
  linked 
  with 
  the 
  secondary 
  winding. 
  

   The 
  greater 
  part 
  of 
  this 
  magnetic 
  field 
  will 
  pass 
  through 
  the 
  

   core. 
  The 
  magnetization 
  of 
  the 
  core 
  will 
  depend 
  upon 
  the 
  

   resultant 
  magnetizing 
  ampere-turns 
  which 
  is 
  equal 
  to 
  the 
  

   primary 
  ampere-turns 
  less 
  the 
  secondary 
  ampere-turns. 
  At 
  

   the 
  moment 
  of 
  break 
  the 
  current 
  in 
  the 
  secondary 
  is 
  at 
  a 
  

   value 
  of 
  say 
  35 
  milliamperes. 
  and 
  the 
  current 
  in 
  the 
  primary 
  

  

  cannot 
  have 
  exceeded 
  J°°^ 
  or 
  5-4 
  amperes. 
  

   61 
  ohms 
  l 
  

  

  