﻿54 
  Mr. 
  E. 
  F. 
  Earhart 
  on 
  the 
  Discharge 
  

  

  When 
  discharge 
  occurred 
  there 
  was 
  evidence 
  of 
  a 
  much 
  

   higher 
  temperature 
  existing 
  at 
  the 
  point 
  when 
  the 
  point 
  was 
  

   negative 
  than 
  for 
  point 
  positive. 
  For 
  distances 
  correspond- 
  

   ing 
  to 
  the 
  flat 
  portion 
  of 
  the 
  curve 
  parallel 
  to 
  the 
  axis 
  of 
  

   abscissae, 
  there 
  was 
  a 
  fusion 
  of 
  the 
  point 
  provided 
  it 
  had 
  been 
  

   negative. 
  A 
  microscopic 
  examination 
  of 
  a 
  negatively 
  dis- 
  

   charged 
  point 
  showed 
  that 
  it 
  had 
  been 
  fused 
  down 
  very 
  

   much 
  in 
  the 
  shape 
  of 
  a 
  mushroom 
  with 
  the 
  convex 
  side 
  

   towards 
  the 
  plane. 
  Under 
  similar 
  conditions 
  for 
  a 
  positive 
  

   point, 
  a 
  fusing 
  temperature 
  had 
  not 
  been 
  attained. 
  A 
  bluish 
  

   discoloration 
  at 
  and 
  near 
  the 
  point 
  showed 
  that 
  a 
  high 
  

   temperature 
  had 
  been 
  reached 
  and 
  the 
  temper 
  drawn. 
  The 
  

   flat 
  portion 
  of 
  the 
  curve 
  parallel 
  to 
  the 
  X 
  axis 
  shows 
  that 
  

   the 
  discharge 
  did 
  not 
  always 
  occur 
  from 
  the 
  point 
  alone. 
  

   When 
  a 
  potential 
  sufficient 
  to 
  produce 
  an 
  ionization 
  of 
  the 
  

   gas 
  is 
  attained, 
  the 
  discharge, 
  or 
  a 
  portion 
  of 
  it 
  at 
  least, 
  will 
  

   occur 
  between 
  the 
  parts 
  removed 
  a 
  distance 
  equal 
  to 
  the 
  

   critical 
  spark-length 
  for 
  the 
  pressure 
  employed. 
  If 
  the 
  

   point 
  lies 
  within 
  this 
  distance, 
  part 
  of 
  the 
  discharge 
  will 
  

   occur 
  from 
  the 
  portion 
  of 
  the 
  needle 
  lying 
  back 
  of 
  the 
  point. 
  

   Hence 
  as 
  the 
  needle 
  is 
  withdrawn 
  no 
  increase 
  in 
  the 
  

   potential-difference 
  needed 
  for 
  the 
  production 
  of 
  a 
  discharge 
  

   will 
  be 
  required 
  until 
  the 
  needle-point 
  is 
  removed 
  to 
  a 
  

   distance 
  exceeding 
  the 
  critical 
  distance. 
  Attention 
  was 
  

   called 
  to 
  this 
  by 
  Carr 
  (Proc. 
  Roy. 
  Soc. 
  xxi. 
  p. 
  374, 
  1903). 
  

   The 
  results 
  published 
  by 
  Hobbs 
  in 
  a 
  paper 
  previously 
  men- 
  

   tioned 
  bring 
  out 
  this 
  point 
  markedly. 
  

  

  The 
  critical 
  distance, 
  if 
  this 
  is 
  the 
  correct 
  interpretation, 
  

   in 
  the 
  case 
  of 
  point 
  and 
  plane 
  is 
  not 
  the 
  same 
  as 
  between 
  

   parallel 
  plates, 
  nor 
  indeed 
  would 
  this 
  be 
  expected 
  owing 
  to 
  

   the 
  difference 
  in 
  the 
  electric 
  fields. 
  My 
  results 
  indicate 
  

   that 
  the 
  critical 
  distance 
  for 
  a 
  pressure 
  of 
  25 
  cm. 
  exceeds 
  

   100 
  wave-lengths 
  of 
  sodium 
  light. 
  

  

  Paschen's 
  law 
  indicates 
  that 
  for 
  a 
  pressure 
  of 
  25 
  cm. 
  the 
  

   critical 
  distance 
  between 
  parallel 
  plates 
  would 
  be 
  40 
  wave- 
  

   lengths. 
  This 
  deduction 
  is 
  based 
  on 
  measurements 
  made 
  by 
  

   Oarr. 
  

  

  In 
  discussing 
  the 
  discharge 
  between 
  point 
  and 
  plane, 
  

   J. 
  J. 
  Thomson 
  makes 
  the 
  following 
  statement 
  (' 
  Conduction 
  

   of 
  Electricity 
  through 
  Gases,' 
  2nd 
  edition, 
  p. 
  511) 
  : 
  — 
  " 
  The 
  

   minimum 
  potential 
  required 
  for 
  the 
  discharge 
  of 
  positive 
  

   electricity 
  from 
  a 
  point 
  is 
  greater 
  than 
  for 
  negative, 
  for 
  the 
  

   minimum 
  potential 
  V 
  is 
  determined 
  by 
  the 
  condition 
  that 
  

   the 
  electric 
  field 
  near 
  the 
  electrode 
  should 
  be 
  strong 
  enough 
  

   to 
  enable 
  the 
  positive 
  ions 
  to 
  produce 
  an 
  adequate 
  stream 
  of 
  

   corpuscles. 
  Now, 
  when 
  the 
  point 
  is 
  the 
  cathode 
  the 
  positive 
  

  

  