﻿Ions 
  in 
  Gases 
  under 
  Different 
  Pressures. 
  287 
  

  

  instant 
  ; 
  while 
  if 
  they 
  were 
  separated 
  from 
  each 
  other 
  along 
  

   the 
  path 
  x 
  y, 
  the 
  rays 
  could 
  be 
  stopped 
  and 
  • 
  the 
  E.M.F. 
  

   applied 
  at 
  a 
  known 
  interval 
  afterward. 
  This 
  interval 
  could 
  

   be 
  varied 
  by 
  varying 
  the 
  distance 
  between 
  the 
  levers 
  L 
  

   and 
  L\ 
  

  

  The 
  time 
  taken 
  by 
  the 
  pendulum 
  to 
  pass 
  from 
  one 
  contact- 
  

   breaker 
  to 
  the 
  other 
  was 
  determined 
  by 
  calibrating 
  the 
  

   pendulum. 
  This 
  was 
  done 
  by 
  attaching 
  a 
  smoked-glass 
  plate 
  

   to 
  the 
  pendulum 
  and 
  obtaining 
  a 
  trace 
  on 
  this 
  plate 
  made 
  by 
  

   a 
  vibrating 
  tuning-fork 
  of 
  known 
  frequency. 
  By 
  this 
  means 
  

   the 
  time 
  taken 
  by 
  the 
  pendulum 
  to 
  pass 
  from 
  one 
  point 
  to 
  

   another 
  could 
  be 
  calculated. 
  The 
  electrode 
  E' 
  was 
  connected, 
  

   through 
  a 
  suitable 
  key 
  at 
  K, 
  to 
  one 
  pair 
  of 
  quadrants 
  of 
  the 
  

   electrometer, 
  while 
  the 
  other 
  pair 
  was 
  connected 
  to 
  earth. 
  

  

  Method 
  of 
  Observation. 
  

  

  The 
  following 
  was 
  the 
  method 
  of 
  observation 
  for 
  deter- 
  

   mining 
  the 
  values 
  of 
  N 
  and 
  n 
  in 
  equation 
  (2). 
  The 
  induction- 
  

   coil 
  circuit 
  was 
  closed 
  and 
  the 
  rays 
  allowed 
  to 
  pass 
  into 
  the 
  

   cylinder 
  for 
  about 
  ten 
  seconds, 
  to 
  insure 
  the 
  ionization 
  

   reaching 
  a 
  maximum. 
  This 
  point 
  was 
  tested 
  by 
  running 
  the 
  

   bulb 
  lor 
  a 
  longer 
  time 
  before 
  applying 
  the 
  E.M.F., 
  and 
  it 
  was 
  

   found 
  that 
  about 
  ten 
  seconds 
  was 
  quite 
  sufficient 
  for 
  the 
  

   ionization 
  to 
  reach 
  a 
  maximum. 
  The 
  pendulum 
  was 
  then 
  

   released, 
  which 
  stopped 
  the 
  rays 
  and 
  applied 
  the 
  E.M.F. 
  to 
  

   E 
  by 
  breaking 
  the 
  contacts. 
  During 
  all 
  this 
  time 
  both 
  pairs 
  

   of 
  quadrants 
  of 
  the 
  electrometer 
  were 
  to 
  earth. 
  In 
  order 
  to 
  

   avoid 
  the 
  violent 
  throw 
  of 
  the 
  electrometer-needle, 
  which 
  

   would 
  occur 
  by 
  induction 
  if 
  the 
  pair 
  of 
  quadrants 
  connected 
  

   to 
  E' 
  were 
  insulated 
  before 
  the 
  E.M.F. 
  was 
  applied 
  to 
  E, 
  

   the 
  connexion 
  between 
  the 
  electrometer 
  and 
  E' 
  was 
  broken 
  

   by 
  the 
  key 
  K 
  until 
  after 
  the 
  lever 
  L' 
  was 
  replaced 
  against 
  Q', 
  

   which 
  was 
  done 
  immediately 
  after 
  the 
  passage 
  of 
  the 
  pendulum. 
  

   After 
  replacing 
  L' 
  3 
  the 
  charge 
  in 
  the 
  plates 
  connected 
  with 
  E', 
  

   due 
  to 
  the 
  ions 
  which 
  passed 
  across 
  on 
  the 
  application 
  of 
  the 
  

   E.M.F. 
  to 
  E, 
  was 
  thrown 
  into 
  the 
  electrometer 
  by 
  closing 
  

   the 
  contact 
  K. 
  The 
  deflexion 
  of 
  the 
  electrometer-needle 
  

   should 
  then 
  be 
  proportional 
  to 
  the 
  number 
  of 
  ions 
  in 
  the 
  gas 
  

   between 
  the 
  electrodes 
  at 
  the 
  instant 
  of 
  the 
  application 
  of 
  the 
  

   E.M.F. 
  to 
  E, 
  if 
  the 
  E.M.F. 
  is 
  large 
  enough 
  to 
  produce 
  

   saturation. 
  The 
  potential-gradient 
  employed 
  in 
  most 
  of 
  the 
  

   observations 
  was 
  about 
  110 
  volts 
  per 
  centimetre. 
  

  

  One 
  of 
  the 
  great 
  difficulties 
  encountered 
  in 
  this 
  investigation 
  

   was 
  the 
  want 
  of 
  constancy 
  in 
  the 
  intensity 
  of 
  the 
  radiation 
  

   given 
  out 
  by 
  the 
  bulb. 
  To 
  obviate 
  this 
  to 
  some 
  extent 
  when 
  

  

  