﻿Velocities 
  of 
  Delta 
  Re 
  s. 
  239 
  

  

  field 
  this 
  charge 
  is 
  far 
  too 
  great 
  to 
  be 
  measured 
  with 
  the 
  

   arrangements 
  described 
  above. 
  "When 
  the 
  case 
  nnd 
  cage 
  are 
  

   both 
  charged 
  to 
  the 
  same 
  negative 
  potential, 
  all 
  the 
  electrons, 
  

   whose 
  kinetic 
  energy 
  is 
  less 
  than 
  the 
  work 
  which 
  this 
  potential 
  

   can 
  do 
  upon 
  an 
  electron, 
  are 
  returned 
  to 
  the 
  source 
  and 
  only 
  

   the 
  swifter 
  ones 
  escape; 
  thus 
  the 
  rate 
  at 
  which 
  the 
  source 
  

   acquires 
  a 
  positive 
  charge 
  is 
  diminished. 
  When, 
  however, 
  

   the 
  negative 
  potential 
  applied 
  to 
  the 
  gauze 
  and 
  case 
  is 
  more 
  

   than 
  about 
  25 
  volts, 
  the 
  source 
  begins 
  to 
  acquire 
  a 
  negative 
  

   charge, 
  which 
  (as 
  the 
  negative 
  potential 
  is 
  increased) 
  soon 
  

   reaches 
  a 
  maximum 
  and 
  then 
  steadily 
  decreases 
  ; 
  this 
  con- 
  

   tinues 
  up 
  to, 
  and 
  beyond, 
  2000 
  volts. 
  This 
  effect 
  has 
  been 
  

   shown 
  to 
  be 
  due 
  to 
  the 
  emission 
  of 
  tertiary 
  electrons 
  by 
  the 
  

   = 
  when 
  struck 
  by 
  the 
  swifter 
  S-rays 
  : 
  the 
  tertiary 
  electrons 
  

   are 
  returned 
  to 
  the 
  source 
  of 
  S-rays 
  by 
  the 
  field, 
  and, 
  as 
  they 
  

   exceed 
  in 
  number 
  the 
  £-rays 
  which 
  produce 
  them, 
  the 
  source 
  

   acquires 
  a 
  resultant 
  negative 
  charge, 
  which 
  falls 
  off, 
  however, 
  

   as 
  more 
  and 
  more 
  ^-electrons 
  are 
  restrained 
  from 
  reaching 
  

   the 
  case*. 
  

  

  It 
  was 
  for 
  the 
  purpose 
  of 
  eliminating, 
  if 
  possible, 
  this 
  

   c 
  implication 
  that 
  the 
  wire-gauze 
  cage 
  was 
  introduced 
  as 
  a 
  

   snbstitnte 
  for 
  the 
  magnetic 
  field 
  mentioned 
  in 
  § 
  2. 
  Retarding 
  

   fields 
  may 
  be 
  set 
  up 
  by 
  putting 
  negative 
  potentials 
  on 
  the 
  

   gauze 
  while 
  the 
  case 
  is 
  kept 
  grounded. 
  As 
  the 
  tertiary 
  

   electrons 
  have 
  small 
  velocities, 
  those 
  which 
  are 
  set 
  up 
  at 
  the 
  

   surface 
  of 
  the 
  case, 
  by 
  the 
  ^-electrons 
  which 
  have 
  surmounted 
  

   the 
  field 
  and 
  passed 
  through 
  the 
  meshes 
  of 
  the 
  gauze, 
  will 
  

  

  Bjet 
  back 
  to 
  the 
  source 
  on 
  account 
  of 
  the 
  field 
  between 
  the 
  

   gauze 
  and 
  the 
  case 
  f. 
  The 
  electric 
  force 
  is 
  much 
  greater 
  

   between 
  gauze 
  and 
  case 
  than 
  between 
  gauze 
  and 
  source 
  

   (though 
  the 
  potential 
  difference 
  is 
  the 
  same) 
  because 
  of 
  the 
  

   smaller 
  distance 
  : 
  thus 
  a 
  considerable 
  fraction 
  of 
  the 
  tertiary 
  

   electrons 
  which 
  originate 
  on 
  the 
  wires 
  of 
  the 
  gauze 
  itself 
  

   will 
  be 
  captured 
  by 
  this 
  field 
  and 
  will 
  not 
  get 
  to 
  the 
  source 
  

   of 
  c-ravs. 
  As 
  the 
  case 
  and 
  source 
  are 
  always 
  kept 
  at 
  zero 
  

   while 
  the 
  potential 
  of 
  the 
  gauze 
  is 
  changed, 
  the 
  sliape 
  of 
  the 
  

  

  - 
  of 
  force, 
  in 
  the 
  neighbourhood 
  of 
  the 
  wires 
  of 
  the 
  gauze 
  

   and 
  within 
  its 
  meshes, 
  will 
  not 
  change, 
  and 
  it 
  seems 
  reasonable 
  

  

  —ume 
  that 
  a 
  nearly 
  constant 
  fraction 
  of 
  the 
  tertiary 
  rays 
  

  

  originating 
  upon 
  the 
  gauze 
  will 
  be 
  captured 
  in 
  this 
  way, 
  and 
  

  

  that 
  the 
  number 
  which 
  get 
  back 
  to 
  the 
  source 
  in 
  any 
  case 
  will 
  

  

  -mall 
  part 
  of 
  the 
  total 
  set 
  up 
  by 
  the 
  o-rays 
  on 
  case 
  and 
  

  

  gether. 
  

  

  * 
  Bumstead 
  and 
  Mc^ouo-an. 
  /. 
  c. 
  § 
  3. 
  

  

  f 
  Tlii- 
  method 
  has 
  been 
  several 
  tames 
  used 
  to 
  prevent 
  the 
  reflexion 
  of 
  

   lectrona 
  in 
  experiments 
  upon 
  cathode 
  rays 
  and 
  on 
  the 
  photoelectric 
  

   effect 
  v. 
  ■ 
  • 
  . 
  / 
  - 
  ',/•. 
  x. 
  p. 
  174 
  (18 
  

  

  