﻿460 
  Dr. 
  J. 
  E. 
  Almy 
  on 
  

  

  By 
  taking 
  a 
  definite 
  spark-gap 
  and 
  gradually 
  increasing 
  

   the 
  applied 
  potential 
  until 
  a 
  discharge, 
  or 
  short-circuit, 
  

   resulted, 
  the 
  following 
  observations 
  were 
  made 
  : 
  

  

  Spark-gap, 
  '15 
  wave-length 
  ; 
  at 
  270 
  volts 
  circuit 
  closed, 
  no 
  discharge 
  was 
  

  

  observable 
  in 
  the 
  microscope. 
  

   Spark-gap, 
  f 
  2 
  wave-length 
  ; 
  at 
  345 
  volts 
  circuit 
  closed, 
  electrodes 
  were 
  

  

  found 
  to 
  be 
  fused 
  together. 
  

   Spark-gap 
  '35 
  wave-length 
  ; 
  at 
  360 
  volts 
  a 
  spark 
  passed, 
  clearly 
  visible, 
  

  

  circuit 
  remained 
  opened 
  for 
  lower 
  potentials. 
  

  

  The 
  evidence 
  seems 
  conclusive 
  that 
  with 
  spark-gaps 
  down 
  

   to 
  at 
  least 
  *3 
  wave-length 
  of 
  Na 
  light, 
  that 
  is, 
  '0000177 
  cm. 
  

   a 
  potential 
  of 
  330 
  volts 
  is 
  not 
  sufficient 
  to 
  produce 
  discharge 
  

   through 
  air 
  at 
  atmospheric 
  pressure, 
  while 
  360 
  volts 
  is 
  sufficient 
  

   to 
  do 
  so. 
  

  

  It 
  is 
  of 
  interest 
  to 
  notice 
  the 
  nature 
  of 
  the 
  discharge 
  with 
  

   these 
  minute 
  spark-gaps. 
  Through 
  the 
  microscope 
  (magni- 
  

   fying 
  about 
  500 
  times), 
  the 
  discharge 
  that 
  occurs 
  with: 
  

   potentials 
  near 
  the 
  " 
  minimum 
  potential 
  " 
  is 
  seen 
  to 
  be 
  a 
  quiet 
  

   luminous 
  glow 
  discharge, 
  the 
  luminescence 
  beginning 
  at 
  a 
  

   point 
  somewhat 
  back 
  from 
  the 
  needle 
  tip, 
  completely 
  surround- 
  

   ing 
  the 
  needle 
  and 
  extending 
  back 
  along 
  the 
  needle 
  (one 
  

   electrode 
  only), 
  exactly 
  similar 
  to 
  the 
  negative 
  glow 
  around 
  

   a 
  cathode 
  in 
  gas 
  at 
  low 
  pressure. 
  But 
  at 
  the 
  tip 
  of 
  the 
  needle 
  

   no 
  luminosity 
  was 
  evident. 
  While 
  with 
  higher 
  potentials 
  a 
  

   brilliant, 
  concentrated, 
  discharge 
  passed 
  in 
  a 
  more 
  limited 
  

   path, 
  appearing 
  in 
  the 
  microscope 
  much 
  like 
  a 
  long 
  arc- 
  

   discharge 
  results. 
  With 
  a 
  gap 
  of 
  several 
  wave-lengths 
  on 
  a 
  

   few 
  occasions 
  the 
  passing 
  of 
  scintillating 
  particles 
  (dust 
  or 
  

   metallic 
  ?) 
  from 
  one 
  electrode 
  to 
  the 
  other 
  was 
  noticed 
  ; 
  once 
  

   or 
  twice 
  this 
  occurred 
  with 
  potentials 
  below 
  the 
  " 
  minimum," 
  

   but 
  could 
  not 
  be 
  repeated. 
  The 
  following 
  observations 
  made 
  

   with 
  steel 
  needles 
  as 
  electrodes 
  are 
  illustrative 
  (p. 
  461). 
  

  

  It 
  seems 
  conclusive 
  from 
  the 
  observations 
  given 
  that, 
  at 
  any 
  

   rate 
  when 
  minute 
  electrodes, 
  comparable 
  in 
  size 
  to 
  the 
  size 
  of 
  the 
  

   spark-gap, 
  are 
  used, 
  there 
  is 
  a 
  " 
  minimum 
  potential 
  " 
  below 
  

   which, 
  no 
  matter 
  what 
  the 
  spark-gap, 
  a 
  true 
  spark-discharge 
  

   does 
  not 
  take 
  place. 
  And 
  there 
  seems 
  large 
  probability 
  that 
  

   Paschen's 
  law 
  of 
  inverse 
  proportionality 
  of 
  spark-length 
  to 
  

   gas 
  pressure 
  is 
  equally 
  applicable 
  to 
  spark-gaps 
  shorter 
  than 
  

   that 
  corresponding 
  to 
  the 
  " 
  minimum 
  potential." 
  Some 
  work 
  

   has 
  been 
  done 
  to 
  verify 
  this 
  3 
  with 
  fair 
  success 
  at 
  pressures 
  

   below 
  atmospheric 
  ; 
  at 
  atmospheric 
  pressures 
  the 
  distances 
  

   concerned 
  are 
  so 
  extremely 
  small 
  that 
  the 
  problem 
  of 
  deter- 
  

   mining 
  the 
  sparking 
  potential, 
  when 
  the 
  discharge 
  is 
  compelled 
  

   to 
  pass 
  through 
  the 
  short 
  path, 
  is 
  one 
  of 
  great 
  difficulty. 
  The 
  

   results 
  obtained 
  will 
  be 
  given 
  in 
  a 
  later 
  paper. 
  

  

  