﻿328 
  Trowbridge 
  and 
  Richards 
  — 
  Temperature 
  and 
  

   Potential 
  differences, 
  between 
  electrodes 
  of 
  spectrum 
  tube. 
  

  

  Hydr 
  

  

  Pressure 
  in 
  

  

  mm 
  

  

  ogen. 
  

  

  Voltage. 
  

  

  Nitrogen. 
  

   Pressure 
  in 
  

  

  VoHage. 
  

  

  7-0 
  

  

  6-0 
  

   4-0 
  

  

  2600 
  (?) 
  

   2100 
  (?) 
  

   1900 
  

  

  8-5 
  

  

  5- 
  

  

  4- 
  

  

  very 
  high 
  

  

  very 
  high 
  

  

  2600 
  (?) 
  

  

  3-5 
  

  

  1500 
  

  

  3' 
  

  

  2100 
  (?) 
  

  

  2-0 
  

  

  1340 
  

  

  2'5 
  

  

  1750 
  

  

  1-5 
  

  

  1260 
  

  

  1-7 
  

  

  1600 
  

  

  1-25 
  

  

  1220 
  

  

  1-4 
  

  

  1410 
  

  

  1-15 
  

  

  1150 
  

  

  1-2 
  

  

  1340 
  

  

  1-00 
  

  

  1100 
  

  

  1-0 
  

  

  1180 
  

  

  •70 
  

  

  1140 
  

  

  0-7 
  

  

  1140 
  

  

  •50 
  

  

  1220 
  

  

  0-6 
  

  

  1080 
  

  

  •13 
  

  

  very 
  high 
  

  

  0-5 
  

  

  0-3 
  

  

  1040 
  

   980 
  

  

  

  

  0*25 
  

  

  1030 
  

  

  

  

  0-13 
  

  

  1700 
  

  

  0-06 
  2800 
  + 
  (?) 
  

  

  Each 
  gas 
  evidently 
  has 
  its 
  minimum 
  of 
  potential 
  difference, 
  

   that 
  of 
  •hydrogen 
  lying 
  at 
  about 
  1 
  millimeter 
  of 
  pressure, 
  and 
  

   that 
  of 
  nitrogen 
  at 
  about 
  3 
  millimeter. 
  These 
  minima, 
  as 
  

   well 
  as 
  the 
  total 
  potential 
  differences, 
  are 
  undoubtedly 
  modified 
  

   by 
  the 
  strength 
  of 
  the 
  current; 
  but 
  the 
  results 
  given 
  above 
  are 
  

   comparable 
  with 
  one 
  another 
  because 
  they 
  were 
  all 
  made 
  under 
  

   the 
  same 
  conditions. 
  Hittorf 
  found 
  a 
  minimum 
  at 
  about 
  0*35 
  

   millimeter 
  for 
  nitrogen, 
  and 
  he 
  pointed 
  out 
  by 
  means 
  of 
  his 
  

   extra 
  electrodes 
  that 
  the 
  fall 
  of 
  potential 
  was 
  very 
  irregular, 
  

   the 
  greater 
  part 
  of 
  it 
  residing 
  at 
  the 
  cathode. 
  His 
  results 
  

   have 
  been 
  confirmed 
  by 
  others, 
  and 
  Wood* 
  has 
  shown 
  that 
  the 
  

   heat 
  evolved 
  at 
  different 
  parts 
  of 
  the 
  tube 
  follows 
  the 
  same 
  

   irregularities 
  as 
  these 
  potential 
  differences. 
  

  

  Neglecting 
  the 
  factors 
  of 
  the 
  potential 
  difference 
  which 
  

   reside 
  at 
  the 
  electrodes, 
  the 
  sum 
  of 
  which 
  increase 
  with 
  the 
  

   exhaustion 
  of 
  the 
  tube, 
  we 
  find 
  that 
  according 
  to 
  Hittorf's 
  

   results 
  the 
  resistance 
  of 
  the 
  gas 
  itself 
  steadily 
  diminishes 
  as 
  the 
  

   exhaustion 
  proceeds. 
  For 
  example, 
  with 
  a 
  current 
  of 
  two 
  

   milliamperes 
  he 
  found 
  a 
  fall 
  of 
  potential 
  of 
  about 
  120 
  volts 
  

   between 
  two 
  parts 
  of 
  the 
  middle 
  of 
  the 
  tube 
  eight 
  centimeters 
  

   apart, 
  the 
  tension 
  of 
  the 
  nitrogen 
  being 
  0*35 
  millimeter. 
  

   When 
  the 
  current 
  was 
  about 
  one 
  milliampere 
  and 
  the 
  tension 
  

   of 
  the 
  gas 
  was 
  only 
  about 
  0*001 
  mm 
  the 
  voltage 
  sank 
  to 
  fifteen. 
  

   These 
  two 
  figures 
  correspond 
  to 
  resistance 
  of 
  60,000 
  ohms 
  and 
  

   15,000 
  ohms 
  respectively, 
  the 
  resistance 
  of 
  the 
  gas 
  diminishing 
  

   as 
  the 
  pressure 
  is 
  decreased. 
  Of 
  course 
  we 
  have 
  no 
  certainty 
  

   as 
  to 
  how 
  T 
  much 
  of 
  this 
  opposition 
  to 
  the 
  current 
  is 
  due 
  to 
  true 
  

  

  * 
  Wied. 
  Ann., 
  lix, 
  238. 
  

  

  