﻿754 
  Prof. 
  0. 
  W. 
  Richardson 
  on 
  the 
  Specific 
  

  

  is 
  determined 
  solely 
  by 
  the 
  magnetic 
  and 
  electric 
  fields. 
  It 
  

   is 
  essential, 
  therefore, 
  that 
  very 
  few 
  of 
  them 
  should 
  collide 
  

   with 
  the 
  atoms 
  of 
  the 
  gas 
  present 
  in 
  the 
  apparatus 
  during 
  

   their 
  passage 
  from 
  the 
  strip 
  to 
  the 
  plates. 
  This 
  condition 
  

   will 
  be 
  satisfied 
  if 
  the 
  mean 
  free 
  path 
  of 
  the 
  ions 
  is 
  con- 
  

   siderably 
  greater 
  than 
  the 
  distance 
  between 
  the 
  two 
  sets 
  of 
  

   plates. 
  In 
  nearly 
  all 
  the 
  experiments 
  the 
  pressure 
  was 
  not 
  

   allowed 
  to 
  rise 
  above 
  '0005 
  mm. 
  At 
  this 
  pressure 
  the 
  mean 
  

   free 
  path 
  of 
  a 
  positive 
  ion 
  is 
  approximately 
  20 
  cms., 
  that 
  of 
  

   a 
  negative 
  electron 
  being 
  four 
  times 
  as 
  great. 
  The 
  distance 
  

   between 
  the 
  plates 
  was 
  always 
  about 
  *5 
  cm., 
  so 
  that 
  the 
  

   required 
  condition 
  was 
  evidently 
  fulfilled. 
  

  

  It 
  has 
  been 
  pointed 
  out 
  on 
  pp. 
  742, 
  743 
  that 
  the 
  first 
  ex- 
  

   periments 
  were 
  made 
  by 
  a 
  method 
  different 
  from 
  that 
  just 
  

   described. 
  The 
  electrode 
  behind 
  the 
  slit 
  was 
  not 
  used, 
  but 
  

   the 
  position 
  of 
  the 
  slit 
  was 
  determined 
  for 
  which 
  each 
  of 
  the 
  

   plates 
  cc 
  (fig. 
  1) 
  received 
  equal 
  charges. 
  This 
  was 
  done 
  for 
  

   different 
  values, 
  both 
  positive 
  and 
  negative, 
  of 
  the 
  magnetic 
  

   field. 
  The 
  apparatus 
  used 
  for 
  this 
  purpose 
  was 
  somewhat 
  

   different 
  from 
  that 
  figured. 
  Each 
  of 
  the 
  plates 
  cc 
  was 
  

   separately 
  mounted 
  on 
  the 
  carriage 
  and 
  insulated, 
  the 
  lower 
  

   of 
  the 
  two 
  instead 
  of 
  1 
  being 
  connected 
  to 
  the 
  mercury 
  

   cup 
  D 
  (fig. 
  2). 
  The 
  cups 
  E 
  and 
  F 
  were 
  connected 
  by 
  wires 
  

   passing 
  down 
  a 
  shielding 
  tube 
  to 
  adjoining 
  quadrants 
  of 
  the 
  

   electrometer 
  H 
  which 
  was 
  insulated. 
  The 
  condenser 
  G 
  and 
  

   plug 
  K 
  2 
  were 
  not 
  used. 
  C 
  and 
  D 
  were 
  connected 
  respectively 
  

   to 
  E 
  and 
  F 
  by 
  means 
  of 
  the 
  plug 
  K 
  l5 
  and 
  the 
  head 
  T 
  was 
  

   turned 
  until 
  a 
  position 
  was 
  found, 
  such 
  that 
  the 
  electrometer 
  

   needle 
  did 
  not 
  deflect 
  on 
  disconnecting 
  the 
  quadrants 
  by 
  

   taking 
  out 
  the 
  plug 
  K 
  3 
  . 
  It 
  was 
  very 
  easy 
  to 
  find 
  two 
  points 
  

   at 
  a 
  very 
  short 
  distance 
  apart, 
  such 
  that 
  the 
  needle 
  deflected 
  

   strongly 
  to 
  the 
  right 
  for 
  one 
  and 
  equally 
  strongly 
  to 
  the 
  left 
  

   for 
  the 
  other. 
  The 
  means 
  of 
  two 
  such 
  points 
  were 
  taken. 
  

   It 
  was 
  found 
  that 
  accurate 
  and 
  consistent 
  results 
  could 
  be 
  

   obtained 
  in 
  this 
  way 
  owing 
  to 
  the 
  method 
  being 
  very 
  sensitive 
  

   to 
  small 
  displacements 
  in 
  the 
  neighbourhood 
  of 
  the 
  equilibrium 
  

   position. 
  The 
  position 
  was 
  found 
  first 
  with 
  the 
  magnetic 
  

   field 
  equal 
  to 
  zero, 
  then 
  when 
  it 
  had 
  a 
  given 
  value 
  in 
  

   one 
  direction, 
  then 
  with 
  zero 
  again, 
  and 
  finally 
  with 
  the 
  

   field 
  reversed. 
  The 
  mean 
  of 
  the 
  two 
  deflexions 
  was 
  

   taken. 
  

  

  A 
  series 
  of 
  measurements 
  made 
  in 
  this 
  way 
  gave 
  the 
  

   following 
  values 
  : 
  — 
  

  

  Magnetic 
  Field 
  H... 
  1930 
  3440 
  4400 
  4900 
  

   Displacement, 
  cms. 
  '105 
  -216 
  "292 
  *347 
  

  

  