﻿S2^ 
  Sir 
  J. 
  J. 
  Thomson 
  on 
  

  

  frequent 
  as 
  the 
  pressure 
  increases, 
  and 
  more 
  and 
  more 
  

   particles 
  are 
  positively 
  charged 
  for 
  only 
  part 
  o£ 
  the 
  time 
  they 
  

   are 
  in 
  the 
  magnetic 
  field. 
  Since 
  they 
  are 
  only 
  deflected 
  

   when 
  charged, 
  the 
  deflexion 
  will 
  vary 
  with 
  the 
  time 
  they 
  

   have 
  been 
  uncharged 
  while 
  passing 
  thiough 
  the 
  magnetic 
  

   field 
  ; 
  and 
  if 
  this 
  time 
  varies 
  continuously 
  the 
  spot 
  will 
  be 
  

   drawn 
  out 
  by 
  the 
  magnet 
  into 
  a 
  continuous 
  band 
  such 
  as 
  is 
  

   represented 
  in 
  fig. 
  2, 
  and 
  which 
  is 
  always 
  observed 
  when 
  the 
  

   pressure 
  is 
  not 
  too 
  low. 
  

  

  An 
  important 
  point 
  as 
  to 
  the 
  determination 
  of 
  ^/m 
  occurs 
  

   in 
  connexion 
  with 
  this 
  effect 
  : 
  the 
  values 
  of 
  e/m 
  got 
  from 
  

   the 
  magnetic 
  and 
  electric 
  deflexions 
  are 
  the 
  average 
  values 
  

   of 
  ejm 
  whilst 
  the 
  system 
  is 
  in 
  the 
  magnetic 
  and 
  electric 
  

   fields. 
  

  

  Since 
  some 
  of 
  the 
  systems 
  are 
  at 
  times 
  electrically 
  neutral, 
  

   the 
  average 
  value 
  of 
  e/m 
  cannot 
  be 
  greater 
  than 
  the 
  true 
  

   value, 
  and 
  it 
  may 
  be 
  considerably 
  less. 
  It 
  may 
  be 
  asked, 
  

   have 
  we 
  any 
  security 
  when 
  we 
  deduce 
  the 
  value 
  of 
  e/m 
  for 
  

   the 
  positive 
  ions 
  from 
  the 
  measurement 
  of 
  the 
  tip 
  of 
  a 
  drawn 
  

   out 
  band 
  such 
  as 
  that 
  in 
  fig. 
  2, 
  that 
  the 
  particle 
  under 
  

   consideration 
  has 
  been 
  charged 
  the 
  whole 
  time 
  it 
  was 
  passing 
  

   tnrough 
  the 
  field 
  ; 
  if 
  this 
  is 
  not 
  the 
  case, 
  the 
  true 
  value 
  of 
  

   e/m 
  may 
  be 
  much 
  greater 
  than 
  the 
  value 
  calculated. 
  

  

  This 
  consideration 
  does 
  not 
  apply 
  to 
  the 
  case 
  when 
  the 
  

   bright 
  spot 
  is 
  deflected 
  as 
  a 
  spot 
  and 
  not 
  as 
  a 
  band 
  ; 
  but 
  a 
  

   great 
  many 
  determinations 
  of 
  e/m 
  have 
  been 
  made 
  when 
  the 
  

   pressure 
  was 
  too 
  high 
  to 
  give 
  spots, 
  and 
  the 
  ends 
  of 
  the 
  bands 
  

   have 
  been 
  used 
  for 
  this 
  purpose. 
  

  

  I 
  have 
  made 
  some 
  experiments 
  to 
  see 
  if 
  in 
  my 
  measure- 
  

   ments 
  any 
  errors 
  have 
  been 
  made 
  from 
  this 
  cause. 
  The 
  

   principle 
  of 
  these 
  experiments 
  was 
  as 
  follows. 
  If 
  the 
  

   particle 
  at 
  the 
  tip 
  of 
  the 
  beam 
  has 
  not 
  been 
  charged 
  the 
  whole 
  

   time, 
  i, 
  e. 
  if 
  it 
  has 
  united 
  with 
  a 
  corpuscle 
  before 
  leaving 
  the 
  

   magnetic 
  field, 
  then 
  the 
  value 
  of 
  e/m 
  determined 
  by 
  this 
  

   method 
  will 
  increase 
  as 
  the 
  length 
  of 
  the 
  path 
  travelled 
  in 
  the 
  

   magnetic 
  field 
  is 
  reduced 
  until 
  it 
  is 
  comparable 
  with 
  the 
  

   length 
  of 
  the 
  path 
  described 
  by 
  the 
  particle 
  whilst 
  it 
  is 
  

   positively 
  charged 
  ; 
  and 
  when 
  the 
  magnetic 
  field 
  is 
  so 
  short 
  

   that 
  the 
  particle 
  can 
  retain 
  its 
  charge 
  for 
  the 
  whole 
  of 
  the 
  

   distance 
  between 
  the 
  poles 
  of 
  the 
  magnet, 
  the 
  value 
  of 
  e/m 
  

   given 
  by 
  this 
  method 
  would 
  rise 
  to 
  a 
  maximum. 
  

  

  Now 
  take 
  two 
  magnetic 
  fields, 
  one 
  short 
  but 
  strong, 
  the 
  

   other 
  long 
  but 
  weak, 
  and 
  adjust 
  the 
  strengths 
  of 
  the 
  fields 
  so 
  

   that 
  if 
  the 
  positive 
  particle 
  retained 
  its 
  charge 
  the 
  whole 
  

   of 
  the 
  time 
  it 
  was 
  in 
  either 
  field, 
  the 
  deflexion 
  of 
  the 
  particle 
  

   would 
  be 
  the 
  same 
  in 
  both 
  cases. 
  If 
  the 
  particles 
  whose 
  

  

  