﻿Ener<jy 
  of 
  the 
  Ions 
  emitted 
  by 
  Hot 
  Bodies. 
  685 
  

  

  small. 
  The 
  ions 
  received 
  by 
  the 
  testing 
  system 
  also 
  were 
  

   emitted 
  by 
  the 
  hottest 
  and 
  most 
  uniformly 
  heated 
  part 
  o£ 
  the 
  

   strip, 
  and 
  irregularities 
  in 
  the 
  electrostatic 
  field 
  due 
  to 
  the 
  

   mounting 
  of 
  the 
  ends 
  of 
  the 
  strip 
  were 
  eliminated. 
  The 
  

   strip 
  was 
  shunted 
  by 
  a 
  high 
  resistance, 
  and 
  the 
  potentials 
  

   were 
  applied 
  at 
  a 
  sliding 
  contact 
  near 
  the 
  mid-point 
  of 
  this. 
  

   The 
  adjustment 
  could 
  be 
  tested 
  by 
  the 
  change 
  in 
  the 
  ther- 
  

   mionic 
  current 
  under 
  zero 
  potential 
  when 
  the 
  heating 
  current 
  

   was 
  reversed*. 
  

  

  The 
  glass 
  tube 
  L 
  was 
  closed 
  at 
  each 
  end 
  by 
  brass 
  plates 
  

   cemented 
  with 
  sealing-wax. 
  The 
  necessary 
  connexions 
  to 
  

   the 
  exterior 
  were 
  made 
  in 
  an 
  obvious 
  kind 
  of 
  way 
  and 
  pre- 
  

   sented 
  no 
  novel 
  features. 
  To 
  exhaust 
  the 
  apparatus 
  a 
  Gaede 
  

   mercury-pump 
  was 
  used 
  and 
  worked 
  continuously. 
  This 
  

   enabled 
  the 
  screw 
  S 
  to 
  be 
  connected 
  to 
  the 
  head 
  outside 
  by 
  

   an 
  axis 
  passing 
  through 
  a 
  mercury-sealed 
  rubber 
  and 
  brass 
  

   joint, 
  in 
  which 
  it 
  turned 
  freely. 
  The 
  air 
  leakage 
  was 
  always 
  

   small 
  compared 
  with 
  the 
  gas 
  evolved 
  when 
  the 
  strip 
  was 
  

   heated. 
  The 
  connexions 
  to 
  the 
  pump 
  and 
  McLeod 
  gauge 
  

   were 
  made 
  with 
  short 
  wide 
  tubes. 
  Even 
  at 
  the 
  highest 
  

   temperature 
  the 
  pressure 
  could 
  always 
  be 
  kept 
  below 
  '004 
  mm., 
  

   and 
  in 
  some 
  of 
  the 
  experiments 
  at 
  lower 
  temperatures 
  the 
  

   pressure 
  registered 
  on 
  the 
  McLeod 
  gauge 
  did 
  not 
  exceed 
  

   •0004 
  mm. 
  The 
  evolution 
  of 
  gas 
  when 
  the 
  strip 
  was 
  heated 
  

   soon 
  reached 
  a 
  steady 
  condition^ 
  when 
  further 
  heating 
  seemed 
  

   to 
  produce 
  little 
  or 
  no 
  diminution 
  in 
  the 
  rate 
  at 
  which 
  it 
  was 
  

   given 
  off. 
  The 
  screw 
  head, 
  which 
  was 
  below 
  the 
  main 
  ap- 
  

   paratus, 
  was 
  provided 
  with 
  a 
  toothed 
  wheel 
  device 
  for 
  regis- 
  

   tering 
  the 
  whole 
  number 
  of 
  turns, 
  and 
  was 
  also 
  divided 
  into 
  

   tenths 
  so 
  that 
  fractions 
  could 
  be 
  measured. 
  One 
  complete 
  

   turn 
  was 
  equal 
  as 
  before 
  to 
  'OGSo 
  cm. 
  

  

  Zero 
  Electric 
  Field. 
  

  

  It 
  was 
  pointed 
  out 
  in 
  the 
  first 
  part 
  of 
  this 
  paper 
  t 
  that 
  in 
  

   the 
  particular 
  case 
  when 
  the 
  plates 
  A 
  and 
  B 
  are 
  at 
  the 
  same 
  

   potential, 
  so 
  that 
  there 
  is 
  no 
  electric 
  field 
  between 
  them, 
  the" 
  

   value 
  of 
  the 
  ratio 
  of 
  the 
  current 
  through 
  the 
  slit 
  to 
  that 
  

   which 
  reaches 
  the 
  plates 
  is 
  given 
  by 
  the 
  very 
  simple 
  relation 
  

  

  where 
  I 
  is 
  the 
  ratio 
  when 
  the 
  slit 
  is 
  in 
  any 
  position 
  distant 
  

   ^ 
  from 
  the 
  central 
  symmetrical 
  position, 
  for 
  which 
  I 
  has 
  the 
  

   maximum 
  value 
  Iq, 
  and 
  z 
  is 
  the 
  distance 
  between 
  the 
  strip 
  

  

  * 
  See 
  Richardson 
  and 
  Brown, 
  Phil. 
  Mag. 
  [6] 
  xvi. 
  p. 
  353 
  (1908) 
  

   t 
  Loc. 
  cit. 
  Equation 
  (14), 
  p. 
  908. 
  

  

  