﻿from 
  an 
  Electrified 
  Point. 
  59 
  

  

  could 
  be 
  the 
  carrier, 
  the 
  potential-gradient 
  would 
  be 
  deter- 
  

   mined 
  by 
  the 
  electrode 
  having 
  the 
  lower 
  gradient 
  irrespective 
  

   o£ 
  its 
  polarity. 
  

  

  In 
  testing 
  this 
  matter, 
  apparatus 
  similar 
  to 
  that 
  employed 
  

   in 
  the 
  point 
  and 
  plane 
  experiment 
  was 
  used, 
  a 
  sphere 
  

   2*54 
  cm. 
  in 
  diameter 
  being 
  substituted 
  for 
  the 
  point. 
  Three 
  

   metals 
  were 
  used 
  in 
  the 
  following 
  permutations, 
  viz. 
  : 
  — 
  

  

  A1 
  + 
  ,A1- 
  Ag 
  + 
  ,Ag- 
  Pt 
  + 
  ,Pt- 
  

   Al 
  + 
  ,Ag- 
  Ag 
  + 
  ,A1- 
  Pt4-,A1- 
  

  

  Al 
  + 
  ,Pt- 
  Ag 
  + 
  ,Pt- 
  Pt 
  + 
  , 
  Ag- 
  

  

  in 
  each 
  o£ 
  these 
  cases 
  the 
  discharge 
  was 
  determined 
  by 
  

   the 
  metal 
  having 
  the 
  lower 
  gradient 
  o£ 
  the 
  pair 
  irrespective 
  

   of 
  its 
  polarity. 
  These 
  results 
  are 
  recorded 
  in 
  the 
  tables 
  VII. 
  

   to 
  XV 
  inclusive. 
  

  

  PL 
  I. 
  fig. 
  5 
  represents 
  graphically 
  the 
  results 
  for 
  Pt 
  and 
  

   Al, 
  i. 
  <?., 
  the 
  extreme 
  case. 
  The 
  curves 
  for 
  Pt-f- 
  Pt— 
  (A) 
  

   and 
  A1 
  + 
  , 
  Al— 
  (B) 
  are 
  similar 
  to 
  those 
  as 
  given 
  by 
  Hobbs 
  ; 
  

   curves 
  and 
  individual 
  readings 
  show 
  good 
  agreement 
  with 
  

   his 
  results. 
  The 
  curves 
  for 
  Pt 
  + 
  5 
  Al— 
  (C) 
  and 
  A1 
  + 
  Pt— 
  

   (D) 
  are 
  the 
  same 
  as 
  (B), 
  the 
  individual 
  readings 
  under 
  the 
  

   stated 
  conditions 
  being 
  indicated. 
  Curves 
  representing 
  the 
  

   same 
  relations 
  for 
  the 
  other 
  combinations 
  are 
  not 
  shown 
  ; 
  

   they 
  indicate, 
  however, 
  the 
  same 
  behaviour, 
  viz. 
  : 
  — 
  that 
  the 
  

   metal 
  having 
  the 
  lower 
  gradient 
  controls 
  the 
  discharge 
  

   irrespective 
  of 
  its 
  polarity. 
  

  

  Summary. 
  

  

  Within 
  the 
  limitations 
  previously 
  described 
  the 
  results 
  

   indicate 
  : 
  — 
  

  

  1st. 
  That 
  the 
  least 
  ionizing 
  potential 
  for 
  point 
  and 
  plane 
  

   is 
  about 
  338 
  volts 
  in 
  air 
  ; 
  a 
  value 
  some 
  12 
  volts 
  less 
  than 
  

   between 
  plane 
  surfaces. 
  

  

  2nd. 
  That 
  the 
  critical 
  distance 
  is 
  greater 
  for 
  point 
  and 
  

   plane 
  than 
  between 
  plane 
  electrodes. 
  

  

  3rd. 
  That 
  the 
  critical 
  potential 
  is 
  less 
  for 
  a 
  negatively 
  

   electrified 
  point 
  than 
  for 
  a 
  positive 
  point. 
  

  

  4th. 
  That 
  the 
  gradient 
  for 
  discharge 
  occurring 
  for 
  

   potential-differences 
  less 
  than 
  the 
  least 
  ionizing 
  potential 
  

   is 
  steeper 
  in 
  the 
  case 
  of 
  point 
  and 
  plane 
  than 
  for 
  plane 
  

   electrodes 
  and 
  is 
  independent 
  of 
  the 
  polarity 
  of 
  the 
  point. 
  

  

  5th. 
  That 
  the 
  carriers 
  for 
  a 
  discharge 
  produced 
  by 
  potential- 
  

   differences 
  less 
  than 
  the 
  least 
  ionizing 
  potential 
  are 
  not 
  

   always 
  negative 
  corpuscles. 
  

  

  Physical 
  Laboratory, 
  

  

  Ohio 
  State 
  Univ., 
  Columbus, 
  Ohio, 
  

  

  Dec. 
  10th, 
  1907. 
  

  

  