﻿Discharge 
  from 
  an 
  Electrified 
  Point. 
  49 
  

  

  potential-difference 
  which 
  is 
  not 
  sufficient 
  to 
  produce 
  an 
  

   ionization 
  of 
  the 
  gas. 
  Measurements 
  made 
  by 
  Kinsley 
  (Phil. 
  

   Mag. 
  [6] 
  ix, 
  p. 
  692, 
  1905), 
  Hobbs.(Phil. 
  Mag. 
  [6] 
  x. 
  p. 
  617, 
  

   1905), 
  and 
  others 
  give 
  approximately 
  the 
  same 
  result. 
  The 
  

   values 
  obtained 
  by 
  Hobbs 
  indicate 
  that 
  the 
  distance 
  through 
  

   which 
  this 
  corpuscular 
  discharge 
  occurs 
  varies 
  with 
  the 
  

   character 
  of 
  the 
  metal 
  electrode. 
  This 
  will 
  be 
  referred 
  to 
  later. 
  

  

  The 
  measurements 
  made 
  by 
  Tamm 
  (Annalen 
  der 
  Phgsik, 
  

   1901, 
  no. 
  10, 
  p. 
  259) 
  were 
  those 
  of 
  a 
  potential-difference 
  

   obtained 
  by 
  operating 
  a 
  static 
  machine. 
  This 
  is 
  an 
  advantage 
  

   in 
  so 
  far 
  as 
  a 
  fine 
  point 
  may 
  be 
  used 
  for 
  successive 
  discharges. 
  

   Sparks 
  of 
  this 
  kind 
  do 
  not 
  involve 
  much 
  energy, 
  and 
  the 
  

   point 
  is 
  but 
  slightly 
  modified 
  by 
  the 
  passage 
  of 
  a 
  single 
  spark. 
  

  

  I 
  proposed 
  in 
  this 
  experiment 
  to 
  use 
  a 
  bank 
  of 
  storage- 
  

   cells 
  as 
  a 
  source 
  of 
  E.M.F. 
  and 
  to 
  measure 
  potential-differences 
  

   by 
  means 
  of 
  a 
  Weston 
  voltmeter 
  placed 
  across 
  the 
  spark-gap. 
  

   A 
  discharge 
  from 
  a 
  needle-point 
  under 
  such 
  conditions 
  

   represents 
  a 
  much 
  greater 
  flow 
  of 
  electrical 
  energy, 
  and 
  

   the 
  point 
  is 
  damaged 
  to 
  such 
  an 
  extent 
  as 
  to 
  make 
  a 
  new 
  one 
  

   necessary. 
  

  

  A 
  series 
  of 
  preliminary 
  measurements 
  between 
  point 
  and 
  

   plane 
  were 
  made 
  with 
  an 
  alternate 
  current 
  source, 
  which 
  

   indicated 
  that 
  with 
  a 
  proper 
  selection 
  of 
  points 
  consistent 
  

   results 
  could 
  be 
  obtained. 
  This 
  preliminary 
  report 
  was 
  made 
  

   before 
  Section 
  B 
  of 
  the 
  A. 
  A. 
  A. 
  S. 
  at 
  the 
  Ithaca 
  meeting 
  

   during 
  the 
  summer 
  of 
  1906. 
  

  

  There 
  are 
  two 
  objections 
  to 
  such 
  a 
  source 
  of 
  potential- 
  

   difference 
  : 
  — 
  -(1) 
  The 
  polarity 
  of 
  the 
  point 
  is 
  unknown 
  when 
  

   discharge 
  occurs. 
  (2) 
  Higher 
  harmonics 
  may 
  be 
  present 
  

   in 
  the 
  potential 
  curve. 
  Discharge 
  would 
  then 
  occur 
  at 
  a 
  

   potential 
  corresponding 
  to 
  the 
  peak 
  of 
  the 
  curve, 
  and 
  the 
  

   square 
  root 
  of 
  mean 
  square 
  value, 
  as 
  given 
  by 
  the 
  voltmeter, 
  

   could 
  not 
  be 
  depended 
  upon 
  to 
  indicate 
  the 
  correct 
  value 
  

   at 
  which 
  discharge 
  occurred. 
  A 
  suitable 
  storage 
  battery 
  will 
  

   at 
  least 
  obviate 
  such 
  difficulties. 
  

  

  The 
  points 
  used 
  in 
  this 
  investigation 
  consisted 
  of 
  No. 
  10 
  

   Sharp 
  needles. 
  A 
  large 
  number 
  were 
  examined 
  by 
  means 
  of 
  

   a 
  microscope, 
  and 
  one 
  which 
  appeared 
  as 
  representing 
  a 
  fair 
  

   average 
  was 
  selected 
  as 
  a 
  master 
  needle. 
  Other 
  needles 
  were 
  

   then 
  examined 
  by 
  means 
  of 
  the 
  microscope 
  and 
  were 
  matched 
  

   against 
  the 
  master 
  needle. 
  

  

  Fig. 
  1 
  indicates 
  the 
  manner 
  of 
  supporting 
  the 
  needle 
  

   and 
  the 
  means 
  used 
  to 
  set 
  it 
  against 
  the 
  plane. 
  The 
  plane 
  

   consisted 
  of 
  a 
  steel 
  disk 
  2*54 
  cm. 
  in 
  diameter, 
  this 
  was 
  

   ground 
  plane 
  but 
  not 
  polished. 
  The 
  plane 
  P 
  was 
  mounted 
  

   ■on 
  and 
  suitably 
  insulated 
  from 
  the 
  movable 
  carriage 
  of 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  16. 
  No. 
  91. 
  July 
  1908. 
  E 
  

  

  