﻿[ 
  48 
  ] 
  

  

  III. 
  Discharge 
  from 
  an 
  Electrified 
  Point 
  and 
  the 
  Nature 
  

   of 
  the 
  Discharge 
  occurring 
  through 
  very 
  small 
  Distances. 
  

   By 
  Robert 
  F. 
  Earhart 
  *. 
  

  

  [Plate 
  I.] 
  Y 
  

  

  Discharge 
  beticeen 
  Point 
  and 
  Plane. 
  

  

  WHEN 
  an 
  electrified 
  point, 
  such 
  as 
  a 
  needle-point, 
  is 
  

   separated 
  from 
  a 
  plane 
  of 
  opposite 
  electrification, 
  a 
  

   discharge 
  will 
  occur 
  i£ 
  the 
  potential-difference 
  between 
  point 
  

   and 
  plane 
  be 
  sufficient. 
  In 
  case 
  the 
  point 
  is 
  positively 
  

   electrified, 
  the 
  potential-difference 
  required 
  for 
  discharge 
  will 
  

   be 
  greater 
  than 
  if 
  the 
  point 
  were 
  negatively 
  electrified. 
  

   This 
  fact 
  has 
  long 
  been 
  known. 
  

  

  A 
  series 
  of 
  measurements 
  by 
  Tamm 
  is 
  quoted 
  by 
  

   J. 
  J. 
  Thomson 
  in 
  ' 
  Conduction 
  of 
  Electricity 
  through 
  Gases,' 
  

   p. 
  499, 
  2nd 
  Edition. 
  The 
  results 
  there 
  recorded 
  for 
  air 
  at 
  

   atmospheric 
  pressure 
  give 
  a 
  minimum 
  potential 
  of 
  2140 
  volts 
  

   for 
  a 
  negative 
  point 
  and 
  a 
  value 
  of 
  3760 
  for 
  a 
  positively 
  

   electrified 
  point. 
  This 
  potential-difference, 
  however, 
  is 
  the 
  

   minimum 
  potential 
  required 
  to 
  cause 
  a 
  perceptible 
  leak 
  

   between 
  a 
  point 
  and 
  a 
  plane 
  several 
  centimetres 
  removed: 
  

   in 
  other 
  words, 
  the 
  least 
  potential-difference 
  required 
  for 
  

   the 
  particular 
  distance 
  shown, 
  not 
  necessarily 
  the 
  least 
  

   potential 
  that 
  will 
  produce 
  a 
  discharge 
  if 
  the 
  distance 
  

   separating 
  the 
  electrodes 
  be 
  decreased. 
  Recent 
  experiments 
  

   have 
  shown 
  that 
  there 
  is 
  a 
  minimum 
  value 
  for 
  potentials 
  

   required 
  to 
  produce 
  an 
  ionization 
  of 
  air 
  or 
  other 
  gas 
  between 
  

   the 
  electrodes, 
  due 
  to 
  the 
  field 
  itself 
  as 
  the 
  ionizing 
  agent. 
  

   This 
  might 
  be 
  called 
  the 
  least 
  ionizing 
  potential. 
  Its 
  value 
  for 
  

   air 
  has 
  been 
  determined 
  by 
  several 
  observers 
  as 
  350 
  volts. 
  

  

  A 
  discharge 
  for 
  potential-difference 
  less 
  than 
  350 
  volts 
  

   may 
  be 
  obtained 
  if 
  the 
  distance 
  between 
  the 
  electrodes 
  is 
  

   very 
  minute. 
  Such 
  discharge 
  at 
  these 
  abnormally 
  small 
  

   distances 
  is 
  probably 
  not 
  due 
  to 
  an 
  ionization 
  of 
  gas 
  between 
  

   the 
  electrodes, 
  but 
  possibly 
  to 
  corpuscles 
  shot 
  off 
  from 
  the 
  

   electrodes. 
  The 
  object 
  of 
  this 
  investigation 
  is 
  to 
  measure 
  

   the 
  potential-difference 
  required 
  to 
  cause 
  a 
  spark 
  to 
  pass 
  

   between 
  a 
  point 
  and 
  plane 
  under 
  certain 
  specified 
  conditions, 
  

   and 
  to 
  make 
  measurements 
  of 
  distance 
  to 
  include 
  the 
  region 
  

   where 
  the 
  projected 
  corpuscles 
  are 
  supposed 
  to 
  carry 
  the 
  

   discharge 
  ; 
  also 
  to 
  examine 
  the 
  nature 
  of 
  this 
  discharge. 
  

  

  Measurements 
  made 
  by 
  the 
  author 
  some 
  years 
  ago 
  (Phil. 
  

  

  Mag. 
  [(»] 
  i. 
  p. 
  147, 
  1901), 
  indicate 
  that 
  for 
  distances 
  less 
  

  

  than 
  three 
  wave-lengths 
  of 
  sodium 
  light 
  a 
  discharge 
  between 
  

  

  a 
  curved 
  electrode 
  and 
  a 
  plane 
  may 
  take 
  place 
  with 
  a 
  

  

  * 
  Commuiiicati'il 
  by 
  the 
  Author. 
  

  

  