﻿Field 
  upon 
  the 
  Electric 
  Discharge 
  through 
  Gases. 
  257 
  

  

  H, 
  contain 
  each 
  a 
  vertical 
  slot. 
  Their 
  front 
  faces 
  are 
  movable, 
  

   and 
  act 
  as 
  clamps 
  to 
  hold 
  a 
  platinum 
  strip, 
  C. 
  Since 
  this 
  

   platinum 
  strip 
  frequently 
  burns 
  out 
  and 
  the 
  deposit 
  on 
  it 
  

   wears 
  off, 
  it 
  is 
  necessary 
  to 
  have 
  a 
  convenient 
  method 
  of 
  

   renewing 
  the 
  cathode. 
  For 
  this 
  purpose 
  the 
  tube, 
  BD, 
  is 
  

   removed 
  and 
  a 
  platinum 
  strip, 
  which 
  is 
  held 
  in 
  a 
  long 
  metal 
  

   finger, 
  is 
  dropped 
  into 
  the 
  slots. 
  The 
  movable 
  faces 
  of 
  the 
  

   heads, 
  G 
  and 
  H, 
  are 
  then 
  brought 
  fast 
  against 
  the 
  strip 
  by 
  

   small 
  set 
  screws. 
  

  

  The 
  cathode 
  strips 
  are 
  made 
  of 
  new 
  platinum, 
  and 
  are 
  

   18 
  mm. 
  long, 
  1*5 
  mm. 
  wide, 
  and 
  O025 
  mm. 
  thick. 
  Before 
  

   Such 
  a 
  strip 
  is 
  put 
  in 
  place 
  it 
  is 
  chemically 
  cleaned 
  and 
  

   handled 
  only 
  with 
  the 
  metal 
  finger 
  ; 
  after 
  insertion 
  in 
  the 
  

   slot, 
  it 
  is 
  kept 
  at 
  a 
  bright 
  red 
  heat 
  a 
  long 
  time 
  by 
  means 
  of 
  

   a 
  current 
  of 
  electricity, 
  and 
  then 
  touched, 
  while 
  still 
  a 
  little 
  

   warm, 
  with 
  a 
  small 
  crystal 
  of 
  calcium 
  nitrate 
  or 
  with 
  the 
  

   point 
  of 
  a 
  fine 
  thread 
  of 
  sealing-wax. 
  The 
  current 
  is 
  after- 
  

   wards 
  increased 
  until 
  oxidation 
  has 
  taken 
  place. 
  The 
  deposit 
  

   has, 
  of 
  course, 
  been 
  placed 
  on 
  the 
  centre 
  of 
  the 
  face 
  of 
  the 
  

   strip 
  away 
  from 
  the 
  anode. 
  The 
  cathode 
  is 
  now 
  ready 
  for 
  

   use, 
  and 
  the 
  tube 
  is 
  re-sealed. 
  

  

  The 
  discharge-current 
  was 
  supplied 
  from 
  a 
  high-tension 
  

   battery 
  of 
  a 
  thousand 
  storage-cells. 
  Most 
  of 
  the 
  results 
  

   were 
  accomplished 
  with 
  a 
  potential 
  of 
  a 
  thousand 
  volts 
  or 
  

   less. 
  The 
  field-magnet 
  was 
  the 
  one 
  described 
  in 
  the 
  previous 
  

   paper, 
  and 
  while 
  a 
  field 
  of 
  about 
  2000 
  units 
  could 
  be 
  obtained 
  

   at 
  a 
  distance 
  of 
  a 
  centimetre 
  and 
  a 
  half 
  from 
  its 
  pole, 
  con- 
  

   siderably 
  less 
  intense 
  fields 
  were 
  commonly 
  used. 
  The 
  

   usual 
  connexions, 
  with 
  dryers 
  of 
  phosphorus 
  pentoxide, 
  were 
  

   made 
  to 
  a 
  Gaede 
  pump, 
  a 
  McLeod 
  gauge, 
  and 
  a 
  device 
  

   with 
  a 
  mercury 
  column 
  seal 
  for 
  admitting 
  dry 
  and 
  purified 
  

   air. 
  

  

  The 
  pressure 
  at 
  which 
  the 
  magnetic 
  rays 
  occur 
  is 
  some- 
  

   what 
  lower 
  in 
  the 
  present 
  arrangement 
  with 
  the 
  Wehnelt 
  

   cathode 
  than 
  in 
  previous 
  experiments. 
  The 
  best 
  conditions 
  

   seem 
  to 
  lie 
  between 
  0'06 
  and 
  0'09 
  mm. 
  with 
  a 
  potential 
  of 
  

   1000 
  volts. 
  While 
  the 
  rays 
  are 
  readily 
  started 
  and 
  main- 
  

   tained, 
  yet 
  there 
  is 
  a 
  tendency 
  for 
  the 
  current 
  to 
  pile 
  up 
  

   and 
  become 
  so 
  intense 
  as 
  to 
  burn 
  out 
  the 
  platinum 
  strip 
  if 
  

   the 
  full 
  current 
  is 
  started 
  suddenly. 
  It 
  is 
  therefore 
  best 
  to 
  

   heat 
  the 
  cathode 
  strip 
  at 
  first 
  to 
  a 
  point 
  considerably 
  below 
  

   the 
  working 
  temperature. 
  If 
  the 
  cathode 
  is 
  then 
  gradually 
  

   heated, 
  a 
  discharge 
  first 
  starts 
  between 
  the 
  anode 
  1 
  and 
  the 
  

   cathode 
  ; 
  additional 
  heating 
  brings 
  the 
  temperature 
  to 
  a 
  

   point 
  where 
  a 
  luminous 
  glow 
  suddenly 
  appears 
  and 
  extends 
  

   from 
  the 
  deposit 
  into 
  the 
  main 
  tube. 
  But 
  frequently, 
  instead 
  

  

  