﻿Minimum 
  Spark 
  Potentials. 
  

  

  461 
  

  

  Spark- 
  Gap 
  

   \ 
  (wave-lengths). 
  

  

  Potential 
  

   (volts). 
  

  

  Remarks. 
  

  

  •4 
  

   •3 
  

  

  345 
  

   345 
  

   350 
  

  

  No 
  discharge. 
  

   Glow 
  discharge. 
  

  

  10 
  

  

  345 
  

   350 
  

  

  No 
  discharge. 
  

   Glow 
  discharge. 
  

  

  1-5 
  

   25 
  

  

  345 
  

   350 
  

   345 
  

   350 
  

  

  No 
  discharge. 
  

   Glow 
  discharge. 
  

   No 
  discharge. 
  

   Glow 
  discharge. 
  

  

  4-0 
  

  

  345 
  

   350 
  

  

  No 
  discharge. 
  

   Glow 
  discharge. 
  

  

  75 
  

  

  320 
  

   350 
  

   440 
  

  

  Scintillation 
  : 
  ceased 
  immediately. 
  

   Glow 
  discharge 
  ; 
  continuous. 
  

   Arc 
  discharge 
  produced. 
  

  

  9-0 
  

   100 
  

   125 
  

   15-0 
  

  

  400 
  

   505 
  

   530 
  

   COO 
  

  

  Arc 
  discharge 
  ; 
  no 
  glow 
  obtained. 
  

  

  Considering 
  the 
  electrostatic 
  force 
  which 
  exists 
  between 
  

   two 
  large 
  electrodes 
  brought 
  within 
  such 
  minute 
  distances 
  as 
  

   were 
  used 
  by 
  Earhart, 
  Shaw, 
  and 
  Hobbs, 
  it 
  seems 
  not 
  wholly 
  

   improbable 
  that 
  the 
  strains 
  sufficient 
  to 
  bring 
  their 
  electrodes 
  

   into 
  contact 
  may 
  have 
  occurred 
  with 
  potentials 
  less 
  than 
  the 
  

   "minimum." 
  So 
  that 
  the 
  potentials 
  that 
  produce 
  a 
  short 
  

   circuit 
  in 
  the 
  spark-gap 
  were 
  simply 
  those 
  required 
  to 
  give 
  

   the 
  requisite 
  displacement 
  of 
  the 
  electrodes. 
  

  

  In 
  connexion 
  with 
  this 
  question 
  of 
  short 
  sparks, 
  attention 
  

   may 
  be 
  called 
  to 
  the 
  fact 
  that 
  Earhart*, 
  and 
  Shawf, 
  in 
  

   recent 
  work 
  upon 
  spark 
  potentials 
  in 
  liquid 
  dielectrics 
  have 
  

   apparently 
  made 
  no 
  attempt 
  to 
  ensure 
  absence 
  of 
  dust 
  or 
  

   other 
  foreign 
  particles 
  from 
  the 
  liquids 
  tested. 
  True, 
  Earhart 
  

   gives 
  as 
  argument 
  for 
  this 
  disregard 
  the 
  fact 
  that 
  the 
  spark- 
  

   potential 
  for 
  the 
  second 
  and 
  succeeding 
  sparks 
  was 
  the 
  same 
  

   as 
  the 
  first, 
  but 
  that 
  is, 
  in 
  truth, 
  evidence 
  showing 
  that 
  the 
  

   dust 
  already 
  j>res8nt 
  does 
  affect 
  the 
  discharge 
  potentials. 
  It 
  

   is 
  well 
  known 
  that 
  the 
  passage 
  of 
  a 
  spark 
  in 
  any 
  insulating- 
  

   oil 
  invariably 
  results 
  in 
  the 
  formation 
  of 
  carbon 
  particles, 
  

   and 
  these 
  particles 
  collect 
  between 
  the 
  electrodes, 
  and 
  it 
  

   hardly 
  seems 
  probable 
  that 
  the 
  presence 
  of 
  particles 
  of 
  so 
  

  

  * 
  Earhart, 
  Phys. 
  Rev. 
  xxiii. 
  p. 
  358 
  (1906). 
  

   f 
  Shaw, 
  Phil. 
  Mag. 
  [6] 
  p. 
  317 
  (1906). 
  

  

  