﻿Positive 
  Ions 
  emitted 
  from 
  various 
  Hot 
  Bodies. 
  655 
  

  

  current 
  -was 
  unchanged 
  ^vhen 
  the 
  direction 
  of 
  the 
  heating 
  

   current 
  was 
  reversed. 
  The 
  heatino- 
  current 
  was 
  reonlated 
  

   by 
  a 
  large 
  and 
  a 
  small 
  resistance 
  ?'2 
  in 
  parallel. 
  The 
  heating 
  

   coil 
  and 
  shunt 
  together 
  formed 
  one 
  arm 
  of 
  a 
  Wheatstone- 
  

   bridge 
  mesh. 
  In 
  some 
  instances 
  the 
  temperature 
  of 
  the 
  

   thermionic 
  radiator 
  was 
  checked 
  by 
  the 
  resistance 
  of 
  the 
  

   platinum 
  heater, 
  and 
  in 
  others 
  the 
  resistance 
  merely 
  indicated 
  

   whether 
  or 
  not 
  conditions 
  were 
  unchanging 
  during 
  the 
  

   observations. 
  

  

  The 
  temperature 
  was 
  ordinarily 
  determined 
  by 
  an 
  optical 
  

   method 
  which 
  within 
  the 
  working 
  limits 
  was 
  perhaps 
  subject 
  

   to 
  a 
  possible 
  error 
  of 
  10 
  per 
  cent. 
  A 
  standardized 
  platinum 
  

   wire 
  of 
  2*1 
  cm. 
  length 
  was 
  placed 
  in 
  a 
  vacuum-tube. 
  This 
  

   wire 
  was 
  in 
  one 
  arm 
  of 
  a 
  Wheatstone-bridge 
  mesh 
  (see 
  

   fig. 
  2). 
  The 
  temperature 
  of 
  this 
  wire 
  was 
  adjusted 
  nntil 
  it 
  

   appeared 
  to 
  the 
  eye 
  to 
  be 
  at 
  the 
  same 
  temperature 
  as 
  the 
  

   thermionic 
  radiator 
  studied. 
  Then 
  the 
  two 
  bodies 
  were 
  

   taken 
  to 
  be 
  at 
  the 
  same 
  temperature, 
  and 
  this 
  temperature 
  was 
  

   read 
  from 
  a 
  temperature-resistance 
  chart 
  for 
  the 
  standardized 
  

   temperature 
  wire. 
  In 
  the 
  calibration, 
  the 
  fiducial 
  points 
  were 
  

   taken 
  at 
  the 
  temperature 
  of 
  the 
  room, 
  at 
  the 
  temperature 
  of 
  

   the 
  melting 
  of 
  potassium 
  sulphate, 
  1066° 
  C, 
  and 
  at 
  the 
  

   temperature 
  of 
  melting 
  platinum, 
  1715^ 
  C. 
  [see 
  paper 
  by 
  

   Richardson 
  and 
  Brown, 
  loc. 
  cit.] 
  . 
  

  

  Results 
  of 
  the 
  Experiments, 
  

  

  After 
  the 
  thermionic 
  body 
  had 
  been 
  properly 
  adjusted 
  in 
  

   position, 
  there 
  was 
  usually 
  not 
  much 
  difficulty 
  in 
  obtaining 
  

   the 
  necessary 
  observations 
  in 
  order 
  to 
  arrive 
  at 
  the 
  value 
  of 
  

   the 
  kinetic 
  energy. 
  In 
  no 
  instance 
  was 
  there 
  any 
  trouble 
  

   arising 
  from 
  a 
  mixture 
  of 
  negative 
  ions 
  with 
  the 
  positive, 
  

   such 
  as 
  was 
  noted 
  in 
  the 
  previous 
  papers. 
  This 
  was 
  mainly 
  

   because 
  the 
  observations 
  were 
  taken 
  at 
  low 
  temperatures 
  and 
  

   before 
  the 
  heating 
  had 
  been 
  continued 
  much 
  over 
  one 
  hour. 
  

   The 
  heating 
  never 
  extended 
  over 
  more 
  than 
  five 
  hours. 
  All 
  

   the 
  materials 
  studied 
  showed 
  a 
  perceptible 
  decay 
  in 
  the 
  

   magnitude 
  of 
  the 
  thermionic 
  current 
  after 
  about 
  30 
  minutes' 
  

   heating. 
  Of 
  course 
  this 
  rate 
  of 
  decay 
  was 
  a 
  function 
  of 
  the 
  

   temperature. 
  Xo 
  attempt 
  was 
  made 
  to 
  determine 
  just 
  how 
  

   it 
  decayed 
  at 
  different 
  temperatures. 
  

  

  The 
  pressure 
  in 
  the 
  different 
  experiments 
  varied 
  between 
  

   0*0001 
  and 
  0*1 
  mm. 
  In 
  a 
  previous 
  paper 
  [loc. 
  cit.'] 
  the 
  

   author 
  showed 
  that 
  within 
  quite 
  a 
  large 
  range 
  the 
  kinetic 
  

   energy 
  of 
  the 
  positive 
  ions 
  from 
  platinum 
  was 
  practically 
  

   independent 
  of 
  the 
  pressure. 
  Consequently, 
  very 
  little 
  

  

  