﻿844 
  Sir 
  J. 
  J. 
  Thomson 
  on 
  

  

  though 
  the 
  pressure 
  is 
  such 
  that 
  they 
  are 
  well 
  developed 
  

   when 
  the 
  magnetic 
  force 
  is 
  absent. 
  

  

  The 
  value 
  of 
  the 
  magnetic 
  force 
  required 
  to 
  get 
  rid 
  of 
  

   these 
  rays 
  is 
  very 
  well 
  defined, 
  a 
  change 
  of 
  a 
  few 
  per 
  cent, 
  

   in 
  the 
  value 
  of 
  this 
  force 
  will 
  entirely 
  alter 
  the 
  appearance 
  

   of 
  the 
  discharge. 
  

  

  It 
  will 
  be 
  seen 
  that 
  when 
  the 
  Ganalstrahlen 
  are 
  accom- 
  

   panied 
  by 
  the 
  other 
  portions 
  of 
  the 
  discharge, 
  there 
  are 
  two 
  

   sets 
  of 
  positive 
  rays 
  present 
  which 
  suffer 
  very 
  difEerent 
  

   deflexions 
  in 
  the 
  magnetic 
  field, 
  the 
  Ganalstrahlen 
  being 
  very 
  

   little 
  deflected, 
  while 
  the 
  portion 
  d 
  is 
  very 
  largely 
  deflected. 
  

   The 
  comparison 
  with 
  the 
  behaviour 
  of 
  the 
  negatively 
  electri- 
  

   fied 
  particles 
  shows 
  I 
  think, 
  however, 
  that 
  this 
  difference 
  is 
  

   due 
  to 
  a 
  difference 
  in 
  the 
  velocity 
  of 
  the 
  particles 
  and 
  not 
  to 
  

   a 
  change 
  in 
  their 
  mass, 
  for 
  we 
  have 
  seen 
  that 
  there 
  are 
  two 
  

   streams 
  of 
  negatively 
  electrified 
  particles 
  which 
  are 
  very 
  

   differently 
  deflected, 
  viz., 
  the 
  rays 
  which 
  constitute 
  the 
  

   pencil 
  c 
  and 
  those 
  forming 
  the 
  pencils 
  a, 
  a' 
  which 
  are 
  bent 
  

   up 
  into 
  small 
  spirals 
  following 
  the 
  lines 
  of 
  magnetic 
  force. 
  

   In 
  the 
  case 
  of 
  the 
  two 
  sets 
  of 
  negative 
  rays, 
  we 
  have 
  clearly 
  to 
  

   do 
  with 
  rays 
  moving 
  with 
  very 
  different 
  velocities 
  ; 
  we 
  may 
  

   regard 
  the 
  more 
  rapidly 
  moving 
  ones 
  as 
  primary 
  rays, 
  the 
  

   slower 
  ones 
  as 
  secondary 
  rays 
  arising 
  from 
  the 
  ionization 
  of 
  

   the 
  gas 
  by 
  the 
  primary 
  in 
  weaker 
  parts 
  of 
  the 
  electric 
  field, 
  

   so 
  that 
  these 
  secondary 
  rays 
  never 
  acquire 
  the 
  velocity 
  of 
  

   the 
  primary, 
  and 
  are 
  therefore 
  more 
  easily 
  deflected. 
  A 
  

   similar 
  explanation 
  applies 
  to 
  the 
  two 
  sets 
  of 
  positive 
  rays, 
  

   the 
  more 
  easily 
  deflected 
  ones 
  arising 
  from 
  the 
  ionization 
  of 
  

   the 
  gas 
  in 
  the 
  weaker 
  parts 
  of 
  the 
  electric 
  field, 
  where 
  they 
  

   have 
  no 
  opportunities 
  of 
  acquiring 
  large 
  velocities. 
  

  

  Note 
  on 
  a 
  Method 
  of 
  Measuring 
  the 
  Effective 
  Magnetic 
  Field 
  

   in 
  the 
  Magnetic 
  Deflexion 
  of 
  the 
  Ganalstrahlen, 
  

  

  If 
  represents 
  the 
  mouth 
  of 
  the 
  tube 
  from 
  which 
  the 
  

   Ganalstrahlen 
  emerge, 
  B 
  the 
  screen 
  supposed 
  to 
  be 
  at 
  right 
  

   angles 
  to 
  the 
  undeflected 
  path 
  of 
  the 
  rays 
  which 
  we 
  take 
  as 
  

   the 
  axis 
  of 
  x, 
  H 
  the 
  magnetic 
  force 
  at 
  any 
  point 
  parallel 
  

   to 
  z^ 
  y 
  the 
  deflexion 
  of 
  the 
  rays 
  due 
  to 
  the 
  magnetic 
  field, 
  

   then 
  

  

  if 
  r 
  is 
  the 
  initial 
  velocity 
  of 
  the 
  rays. 
  As 
  the 
  deflexion 
  is 
  

  

  