﻿224 
  Electrical 
  Conductivities 
  produced 
  by 
  Negative 
  Ions. 
  

  

  small. 
  On 
  the 
  other 
  hand, 
  the 
  positive 
  ions 
  do 
  not 
  appear 
  

   to 
  have 
  the 
  same 
  effect. 
  

  

  17. 
  Professor 
  J. 
  J. 
  Thomson 
  (Phil. 
  Mag. 
  Dec. 
  1899) 
  

   deduces 
  from 
  experiments 
  with 
  ultra-violet 
  light 
  3 
  x 
  10 
  -26 
  

   of 
  a 
  gramme 
  as 
  the 
  mass 
  of 
  a 
  negative 
  ion. 
  Professor 
  

   Townsend 
  (Phil. 
  Trans. 
  1899) 
  finds 
  4*5 
  X 
  10~ 
  24 
  grammes 
  as 
  

   the 
  weight 
  of 
  a 
  molecule 
  of 
  hydrogen, 
  a 
  determination 
  which 
  

   makes 
  66 
  X 
  10~ 
  23 
  the 
  corresponding 
  weight 
  for 
  air. 
  This 
  is 
  

   more 
  than 
  three 
  times 
  the 
  number 
  given 
  by 
  Meyer 
  (Kinetic 
  

   Theory 
  of 
  Gases, 
  art. 
  121). 
  Thus, 
  if 
  we 
  assume 
  that 
  the 
  

   negative 
  ions 
  produced 
  by 
  the 
  Rontgen 
  rays 
  are 
  identical 
  

   with 
  those 
  obtained 
  by 
  means 
  of 
  ultra-violet 
  light, 
  the 
  mole- 
  

   cules 
  of 
  air 
  are 
  2200 
  times 
  the 
  size 
  of 
  the 
  negative 
  ion 
  ac- 
  

   cording 
  to 
  these 
  numbers. 
  I£ 
  the 
  positive 
  ion 
  is 
  what 
  remains 
  

   of 
  the 
  molecule 
  alter 
  the 
  negative 
  is 
  detached 
  from 
  it, 
  it 
  

   follows 
  that 
  the 
  positive 
  ion 
  does 
  not 
  materially 
  differ 
  in 
  

   mass 
  from 
  the 
  ordinary 
  molecule 
  of 
  air. 
  In 
  any 
  case 
  it 
  

   appears 
  evident 
  that 
  it 
  far 
  exceeds 
  in 
  magnitude 
  the 
  negative 
  

   ion. 
  Therefore 
  the 
  mean 
  path 
  of 
  the 
  latter 
  exceeds 
  that 
  of 
  

   the 
  former. 
  In 
  fact, 
  Prof. 
  Townsend 
  (Phil. 
  Mag. 
  Feb. 
  1901) 
  

   finds 
  about 
  *47 
  mm. 
  as 
  the 
  path 
  of 
  the 
  negative 
  ion 
  in 
  1 
  mm. 
  

   pressure 
  of 
  air. 
  The 
  corresponding 
  path 
  of 
  the 
  positive 
  ion 
  

   assumed 
  to 
  be 
  equal 
  to 
  that 
  of 
  the 
  molecule 
  of 
  air, 
  is 
  about 
  

   '094 
  of 
  a 
  millimetre. 
  This 
  must 
  be 
  multiplied 
  by 
  \/i 
  

   when 
  the 
  ion 
  is 
  moving 
  under 
  electric 
  force, 
  its 
  velocity 
  far 
  

   exceeding' 
  then 
  the 
  velocitv 
  of 
  agitation 
  of 
  air. 
  Its 
  free 
  

   path 
  then 
  becomes 
  *13 
  mm. 
  If 
  e 
  is 
  the 
  charge 
  on 
  a 
  positive 
  or 
  

   negative 
  ion, 
  the 
  energy 
  acquired 
  between 
  successive 
  impacts 
  

   in 
  a 
  field 
  of 
  electric 
  force, 
  X 
  at 
  any 
  point, 
  is 
  e\Xds, 
  the 
  in- 
  

   tegral 
  being 
  taken 
  along 
  the 
  free 
  path. 
  Thus 
  there 
  will 
  be 
  

   a 
  marked 
  difference, 
  so 
  far 
  as 
  energy 
  is 
  concerned, 
  between 
  

   the 
  impacts 
  of 
  the 
  positive 
  and 
  negative 
  ions. 
  It 
  appears, 
  

   however, 
  from 
  observations 
  of 
  sparking 
  points 
  during 
  these 
  

   experiments, 
  that 
  this 
  difference 
  of 
  energy 
  before 
  impact 
  

   does 
  not 
  suffice 
  to 
  account 
  for 
  the 
  difference 
  of 
  the 
  effect 
  of 
  

   impact. 
  But 
  this 
  is, 
  of 
  course, 
  only 
  to 
  be 
  expected. 
  

  

  3 
  8. 
  It 
  is 
  proved 
  above 
  that 
  the 
  negative 
  current 
  is 
  always 
  

   greater 
  than 
  the 
  positive 
  when 
  only 
  negative 
  ions 
  generate 
  

   others 
  by 
  collision. 
  Nevertheless^ 
  observation 
  seems 
  to 
  show 
  

   that 
  sparking 
  occurs 
  sooner 
  when 
  the 
  aluminium 
  cylinder 
  is 
  

   positive 
  than 
  when 
  it 
  is 
  negative. 
  Moreover, 
  it 
  is 
  possible 
  

   to 
  find 
  for 
  this 
  cylinder 
  a 
  potential 
  which 
  upsets 
  the 
  stability 
  

   of 
  the 
  gas, 
  so 
  that 
  a 
  current 
  starts 
  across 
  to 
  the 
  wire 
  either 
  

   spontaneously 
  or 
  when 
  the 
  slightest 
  amount 
  of 
  ionization 
  is 
  

   supplied 
  by 
  the 
  Rontgen 
  rays. 
  This 
  unstable 
  condition 
  

  

  