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  XXIII. 
  On 
  Electric 
  Currents 
  through 
  Air 
  Films. 
  

   By 
  Prof. 
  A. 
  Anderson 
  *. 
  

  

  IN 
  the 
  Philosophical 
  Magazine 
  for 
  May 
  1912 
  there 
  is 
  a 
  

   short 
  paper 
  in 
  which 
  some 
  experiments 
  on 
  electric 
  

   currents 
  in 
  air 
  at 
  atmospheric 
  pressure 
  are 
  referred 
  to. 
  

   Mr. 
  H. 
  N. 
  Morrison, 
  who 
  is 
  now 
  working 
  at 
  the 
  subject, 
  

   has 
  obtained 
  further 
  results, 
  some 
  of 
  which, 
  I 
  venture 
  to 
  

   think, 
  are 
  of 
  sufficient 
  importance 
  to 
  be 
  communicated 
  before 
  

   all 
  of 
  them 
  can 
  be 
  presented 
  in 
  their 
  final 
  form. 
  

  

  Ficr. 
  1. 
  

  

  ^ 
  

  

  £tCC 
  77,- 
  QM 
  F 
  TFP&V////V& 
  

  

  Two 
  brass 
  bars, 
  A 
  and 
  B, 
  fig. 
  1, 
  with 
  carefully 
  worked 
  

   plane 
  ends 
  each 
  of 
  area 
  1*15 
  sq. 
  cm., 
  are 
  mounted 
  firmly 
  

   on 
  paraffin 
  blocks 
  stuck 
  on 
  a 
  copper 
  plate, 
  so 
  as 
  to 
  include 
  

   between 
  them 
  a 
  rectangular 
  film 
  of 
  air 
  of 
  a 
  uniform 
  thickness 
  

   of 
  about 
  "002 
  cm. 
  This 
  apparatus 
  is 
  placed 
  inside 
  an 
  earthed 
  

   copper 
  box 
  which 
  can 
  be 
  made 
  nearly 
  air-tight. 
  Air, 
  after 
  

   having 
  been 
  passed 
  through 
  drying-tubes 
  and 
  tubes 
  containing 
  

   closely 
  packed 
  glass-wool, 
  was 
  passed 
  slowly 
  at 
  intervals 
  

   through 
  this 
  box 
  for 
  several 
  days, 
  and 
  no 
  air 
  was 
  afterwards 
  

   admitted 
  which 
  had 
  not 
  been 
  similarly 
  treated. 
  If 
  B 
  be 
  

   connected 
  to 
  one 
  of 
  the 
  poles 
  of 
  a 
  battery 
  of 
  cells 
  and 
  A 
  

   to 
  one 
  of 
  the 
  terminals 
  of 
  a 
  quadrant 
  electrometer, 
  the 
  

   electrometer 
  will 
  charge 
  up 
  and 
  the 
  current 
  can 
  be 
  measured 
  

   easily. 
  If 
  A 
  and 
  B 
  are 
  different 
  metals 
  and 
  B 
  earthed, 
  

   the 
  electrometer 
  will 
  also 
  charge 
  up, 
  and 
  at 
  a 
  surprisingly 
  

   quick 
  rate, 
  the 
  final 
  deflexion 
  giving 
  the 
  contact 
  difference 
  of 
  

   potential. 
  Indeed 
  this 
  is 
  the 
  easiest 
  method 
  of 
  measuring 
  

   contact 
  differences, 
  no 
  special 
  ionizing 
  agent 
  being 
  necessary. 
  

  

  Currents 
  between 
  two 
  brass 
  bars 
  as 
  above 
  described 
  are 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  