﻿■Electric 
  Wind 
  from 
  a 
  Discharging 
  Point. 
  699 
  

  

  They 
  are 
  due 
  to 
  an 
  effect 
  which 
  is 
  undoubtedly 
  present 
  in 
  all 
  

   cases 
  at 
  still 
  lower 
  pressures 
  and 
  which 
  prevents 
  the 
  extension 
  

   of 
  the 
  experiments 
  to 
  low 
  atmospheric 
  pressures. 
  The 
  linear 
  

   relationship 
  between 
  p 
  and 
  z 
  only 
  holds 
  i£ 
  the 
  proportion 
  of 
  

   current 
  carried 
  by 
  discharge 
  (i£ 
  any) 
  from 
  the 
  plate 
  does 
  not 
  

   alter 
  as 
  z 
  is 
  altered. 
  At 
  low 
  pressures 
  back 
  discharge 
  is 
  

   considerable 
  and 
  appears 
  to 
  increase 
  with 
  z. 
  This 
  has 
  the 
  

   effect 
  of 
  giving 
  the 
  line 
  too 
  small 
  a 
  slope 
  and 
  too 
  small 
  a 
  

   value 
  for 
  z 
  . 
  In 
  every 
  case 
  in 
  which 
  these 
  low 
  readings 
  were 
  

   observed 
  it 
  was 
  found 
  that 
  the 
  discharge 
  was 
  luminous 
  when 
  

   viewed 
  in 
  the 
  dark. 
  This 
  effect 
  seems 
  to 
  set 
  in 
  more 
  readily 
  

   with 
  positive 
  discharge 
  than 
  with 
  negative 
  ; 
  thus 
  no 
  readings 
  

   in 
  positive 
  discharge 
  below 
  11 
  = 
  28 
  cm. 
  could 
  be 
  obtained 
  

   without 
  it. 
  In 
  00 
  2 
  it 
  seemed 
  to 
  be 
  present 
  at 
  all 
  pressures 
  

   in 
  the 
  case 
  of 
  positive 
  discharge, 
  and 
  no 
  values 
  of 
  z 
  to 
  which 
  

   any 
  meaning 
  could 
  be 
  attached 
  were 
  obtained. 
  

  

  The 
  values 
  of 
  z 
  obtained 
  at 
  atmospheric 
  pressure 
  in 
  air 
  

   were 
  practically 
  the 
  same 
  for 
  both 
  signs 
  of 
  discharge, 
  a 
  

   result 
  previously 
  obtained 
  by 
  Tyndall 
  ; 
  the 
  actual 
  values, 
  

   *19 
  cm. 
  and 
  '195 
  cm. 
  for 
  positive 
  and 
  negative 
  discharge 
  

   respectively, 
  are 
  however 
  smaller 
  than 
  his 
  values. 
  This 
  

   may 
  be 
  due 
  to 
  the 
  fact 
  that 
  in 
  his 
  work 
  the 
  point 
  was 
  sheathed 
  

   more 
  closely 
  to 
  the 
  end, 
  thus 
  tending 
  to 
  make 
  the 
  glow 
  at 
  

   the 
  point 
  more 
  of 
  a 
  brush 
  in 
  form. 
  The 
  value 
  of 
  z 
  at 
  

   atmospheric 
  pressure 
  in 
  C0 
  2 
  was 
  *46 
  cm,, 
  which 
  is 
  consi- 
  

   derably 
  greater 
  than 
  that 
  in 
  air. 
  

  

  The 
  results 
  given 
  below 
  supply 
  some 
  further 
  information. 
  

   Thus 
  Table 
  I. 
  shows 
  the 
  ratio 
  (r) 
  of 
  the 
  slopes 
  of 
  the 
  wind- 
  

   pressure-distance 
  curves 
  for 
  positive 
  and 
  negative 
  discharge 
  

   at 
  different 
  atmospheric 
  pressures 
  II. 
  

  

  Table 
  I. 
  

  

  n. 
  

  

  r. 
  

  

  75-5 
  

  

  1-25 
  

  

  65-9 
  

  

  1-25 
  

  

  63-9 
  

  

  1-24 
  

  

  56-7 
  

  

  1-22 
  

  

  501 
  

  

  1-26 
  

  

  39-4 
  

  

  1-25 
  

  

  It 
  will 
  be 
  seen 
  that 
  r 
  is 
  practically 
  constant. 
  This 
  is 
  in 
  

   agreement 
  with 
  the 
  well-known 
  fact 
  that 
  within 
  these 
  limits 
  

   of 
  pressure 
  the 
  velocities 
  of 
  the 
  ions 
  are 
  inversely 
  proportional 
  

  

  