﻿Discharges 
  in 
  High 
  Vacua. 
  

  

  903 
  

  

  glowing 
  metal, 
  or 
  other 
  active 
  cathode, 
  is 
  subjected 
  to 
  a 
  

   magnetic 
  force 
  acting 
  in 
  a 
  direction 
  at 
  right 
  angles 
  to 
  that 
  

   of 
  the 
  electric 
  field. 
  In 
  investigating 
  the 
  ionization 
  pro- 
  

   duced 
  by 
  heated 
  substances, 
  the 
  writer 
  has 
  occasionally 
  tested 
  

   the 
  effect 
  of 
  a 
  magnetic 
  field 
  upon 
  the 
  negative 
  emission 
  

   with 
  different 
  gas 
  pressures 
  in 
  the 
  discharge-tube. 
  The 
  

   apparatus 
  used 
  has 
  been 
  described 
  in 
  a 
  recent 
  paper 
  *. 
  The 
  

   cathode 
  was 
  a 
  strip 
  of 
  platinum 
  foil 
  about 
  2 
  mm. 
  wide 
  and 
  

   2 
  cm. 
  long, 
  which 
  was 
  covered 
  with 
  the 
  material 
  to 
  be 
  

   tested, 
  and 
  heated 
  electrically. 
  This 
  was 
  situated 
  midway 
  

   between 
  two 
  parallel 
  plates, 
  placed 
  1 
  cm. 
  apart, 
  which 
  

   together 
  formed 
  the 
  anode. 
  The 
  magnetic 
  field 
  was 
  applied 
  

   so 
  that 
  its 
  direction 
  was 
  parallel 
  to 
  the 
  planes 
  of 
  the 
  strip 
  

   and 
  plates, 
  and 
  at 
  right 
  angles, 
  therefore, 
  to 
  the 
  direction 
  of 
  

   the 
  electric 
  field. 
  It 
  was 
  found 
  that 
  the 
  effect 
  of 
  a 
  magnetic 
  

   field 
  upon 
  the 
  current 
  depended 
  not 
  only 
  upon 
  its 
  magnitude, 
  

   but 
  also 
  upon 
  the 
  gas 
  pressure 
  in 
  the 
  apparatus, 
  and 
  the 
  

   applied 
  potential 
  difference 
  between 
  the 
  electrodes. 
  At 
  

   certain 
  pressures 
  and 
  with 
  a 
  large 
  potential 
  difference 
  the 
  

   application 
  of 
  a 
  weak 
  magnetic 
  field 
  increased 
  the 
  current 
  

   across 
  the 
  tube. 
  If 
  then, 
  the 
  gas 
  pressure 
  were 
  kept 
  

   constant 
  and 
  the 
  voltage 
  lowered, 
  a 
  point 
  was 
  reached 
  where 
  

   the 
  magnetic 
  force 
  had 
  no 
  effect, 
  and 
  on 
  still 
  further 
  de- 
  

   creasing 
  the 
  voltage, 
  the 
  current 
  through 
  the 
  discharge-tube 
  

   was 
  reduced 
  when 
  the 
  magnetic 
  field 
  was 
  applied. 
  The 
  

   following 
  numbers, 
  which 
  were 
  obtained 
  with 
  a 
  thorium 
  

   oxide 
  cathode, 
  will 
  serve 
  to 
  illustrate 
  this. 
  Instead 
  of 
  giving 
  

   the 
  actual 
  values 
  of 
  the 
  currents 
  through 
  the 
  discharge-tube, 
  

   the 
  table 
  contains 
  the 
  percentage 
  increase 
  or 
  decrease 
  of 
  the 
  

   current 
  produced 
  by 
  applying 
  the 
  magnetic 
  force. 
  

  

  

  Effect 
  of 
  the 
  application 
  of 
  the 
  magnetic 
  field 
  upon 
  the 
  current. 
  

  

  Pressure 
  

   mm. 
  

  

  

  

  

  

  

  

  

  -220 
  volts. 
  

  

  -40 
  volts. 
  

  

  -30 
  volts 
  

  

  —10 
  volts. 
  

  

  -2 
  volts. 
  

  

  1-390 
  

  

  Increased 
  

   10 
  per 
  cent. 
  

  

  Decreased 
  

   6 
  per 
  cent. 
  

  

  

  

  

  •191 
  

  

  Increased 
  

  

  Increased 
  

  

  

  Decreased 
  

  

  Decreased 
  

  

  

  58 
  per 
  cent. 
  

  

  2 
  per 
  cent. 
  

  

  

  32 
  per 
  cent. 
  62 
  per 
  cent. 
  

  

  •015 
  

  

  Increased 
  

  

  No 
  effect. 
  

  

  Decreased 
  

  

  Decreased 
  

  

  Decreased 
  

  

  

  12 
  per 
  cent. 
  

  

  

  4 
  per 
  cent. 
  

  

  39 
  per 
  cent. 
  

  

  75 
  per 
  cent. 
  

  

  •002 
  

  

  No 
  effect. 
  

  

  Decreased 
  

  

  Decreased 
  

  

  Decreased 
  

  

  

  

  

  2 
  per 
  cent. 
  

  

  8 
  per 
  cent. 
  

  

  44 
  per 
  cent. 
  

  

  

  * 
  Proc. 
  Roy. 
  Soc. 
  A. 
  vol. 
  lxxxviii. 
  p. 
  117 
  (1913). 
  

  

  