﻿66S 
  Prof. 
  J. 
  J. 
  Thomson 
  

  

  on 
  

  

  the 
  Canalstrahlen 
  which 
  have 
  moved 
  up 
  to 
  and 
  then 
  passed 
  

   through 
  the 
  cathode. 
  This 
  result 
  is 
  remarkable, 
  for 
  in 
  the 
  

   latter 
  case 
  the 
  action 
  of 
  the 
  electric 
  field 
  which 
  produces 
  the 
  

   discharge 
  in 
  the 
  tube 
  would 
  increase 
  the 
  velocity 
  of 
  a 
  

   positively 
  charged 
  particle, 
  while 
  in 
  the 
  former 
  it 
  would 
  

   diminish 
  it. 
  The 
  fact 
  that 
  the 
  velocities 
  in 
  the 
  two 
  cases 
  are 
  

   very 
  much 
  the 
  same 
  suggests, 
  at 
  first, 
  the 
  suspicion 
  that 
  the 
  

   electric 
  field 
  may 
  not 
  be 
  accountable 
  for 
  any 
  considerable 
  

   portion 
  of 
  the 
  velocity 
  in 
  either 
  case, 
  and 
  that 
  perhaps 
  the 
  

   particles 
  forming 
  these 
  rays 
  may, 
  like 
  the 
  a 
  particles 
  given 
  

   out 
  by 
  radioactive 
  substances, 
  start 
  with 
  a 
  high 
  initial 
  velocity, 
  

   much 
  higher 
  than 
  they 
  could 
  acquire 
  under 
  the 
  electric 
  field. 
  

   If 
  we 
  refer 
  to 
  the 
  table 
  of 
  velocities 
  given 
  on 
  pages 
  66*2-3, 
  we 
  

   shall 
  see 
  that 
  there 
  is 
  not, 
  in 
  the 
  different 
  experiments, 
  much 
  

   variation 
  in 
  the 
  velocity 
  of 
  the 
  particles 
  ; 
  these 
  are 
  always 
  

   about 
  2 
  x 
  10 
  8 
  cm./sec, 
  and 
  could 
  be 
  generated 
  by 
  a 
  fall 
  

   through 
  a 
  potential-difference 
  of 
  about 
  20,000 
  volts. 
  We 
  

   must 
  not, 
  however, 
  attach 
  too 
  much 
  importance 
  to 
  the 
  

   constancy 
  of 
  the 
  velocity, 
  for 
  the 
  range 
  of 
  pressure 
  over 
  which 
  

   we 
  can 
  make 
  accurate 
  observations 
  on 
  the 
  retrograde 
  rays 
  is 
  

   very 
  limited. 
  For 
  when 
  the 
  pressure 
  gets 
  very 
  low, 
  and 
  the 
  

   discharge 
  requires 
  a 
  high 
  potential-difference 
  to 
  send 
  it 
  

   through 
  the 
  tube, 
  the 
  rays 
  are 
  not 
  bright 
  enough 
  to 
  be 
  observed; 
  

   while 
  if 
  the 
  pressure 
  is 
  more 
  than 
  a 
  small 
  fraction 
  of 
  a 
  milli- 
  

   metre, 
  the 
  rays 
  either 
  do 
  not 
  reach 
  the 
  screen, 
  or 
  when 
  they 
  

   do 
  reach 
  it 
  are 
  so 
  diffuse 
  that 
  the 
  phosphorescent 
  patch 
  is 
  not 
  

   definite 
  enough 
  for 
  its 
  position 
  to 
  be 
  measured 
  with 
  accuracy. 
  

   And 
  although 
  even 
  rough 
  measurements 
  show 
  that 
  at 
  these 
  

   high 
  pressures 
  the 
  velocity 
  of 
  the 
  particles 
  is 
  less 
  than 
  when 
  

   the 
  pressure 
  is 
  low, 
  we 
  should 
  not 
  be 
  justified 
  without 
  further 
  

   evidence 
  in 
  concluding 
  that 
  the 
  initial 
  velocity 
  was 
  less, 
  for 
  

   before 
  reaching 
  the 
  screen 
  the 
  rays 
  have 
  had 
  to 
  make 
  a 
  long 
  

   journey 
  through 
  the 
  gas, 
  and 
  if 
  the 
  pressure 
  is 
  high, 
  they 
  

   will 
  in 
  their 
  journey 
  lose 
  more 
  of 
  their 
  velocity 
  than 
  when 
  

   the 
  pressure 
  is 
  low. 
  

  

  To 
  test 
  whether 
  or 
  not 
  the 
  velocity 
  of 
  the 
  rays 
  was 
  due 
  to 
  

   the 
  electric 
  field, 
  I 
  tried 
  the 
  following 
  experiments 
  : 
  — 
  

  

  In 
  the 
  first 
  experiment, 
  a 
  piece 
  of 
  wire 
  gauze 
  was 
  placed 
  

   about 
  2 
  mm. 
  in 
  front 
  of 
  the 
  perforated 
  electrode 
  of 
  the 
  tube 
  

   represented 
  in 
  fig. 
  2 
  and 
  well 
  insulated 
  from 
  it 
  ; 
  the 
  gauze 
  

   was 
  used 
  as 
  the 
  cathode 
  and 
  measurements 
  of 
  the 
  velocity 
  of 
  

   the 
  particles 
  were 
  made 
  (1) 
  when 
  the 
  perforated 
  electrode 
  

   was 
  connected 
  with 
  the 
  gauze, 
  (2) 
  when 
  it 
  was 
  connected 
  

   with 
  the 
  anode. 
  In 
  the 
  second 
  case, 
  a 
  particle 
  if 
  it 
  retained 
  

   its 
  charge 
  during 
  the 
  whole 
  of 
  its 
  path, 
  would 
  in 
  its 
  journey 
  

   between 
  the 
  gauze 
  and 
  the 
  perforated 
  electrode 
  be 
  at 
  least 
  as 
  

  

  