﻿Rotation 
  of 
  Electric 
  Discharge. 
  535 
  

  

  curve 
  bends 
  away 
  from 
  the 
  straight 
  line 
  BC, 
  and 
  at 
  this 
  

   point 
  (somewhere 
  about 
  C) 
  it 
  appears 
  that 
  the 
  above 
  law 
  

   ceases 
  to 
  hold. 
  Ultimately 
  it 
  bends 
  round, 
  as 
  is 
  also 
  a 
  priori 
  

   evident 
  from 
  the 
  fact 
  that 
  at 
  a 
  very 
  low 
  pressure 
  the 
  resistance 
  

   to 
  the 
  passage 
  of 
  discharge 
  is 
  very 
  great. 
  With 
  the 
  method 
  

   of 
  spark 
  distance, 
  it 
  is 
  difficult 
  to 
  trace 
  this 
  portion 
  of 
  the 
  

   curve 
  ; 
  I 
  have 
  accordingly 
  made 
  use 
  of 
  a 
  different 
  method 
  

   (fig. 
  8, 
  III., 
  p. 
  536) 
  for 
  the 
  purpose. 
  

  

  13. 
  If 
  we 
  admit 
  that 
  pressure 
  x 
  spark-length 
  is 
  a 
  linear 
  

   function 
  of 
  the 
  corresponding 
  potential-difference 
  between 
  

   the 
  electrodes, 
  the 
  fact 
  that 
  AB, 
  BC 
  are 
  approximately 
  

   straight 
  indicates 
  that 
  the 
  average 
  length 
  of 
  the 
  path 
  of 
  

   the 
  discharge 
  is 
  very 
  nearly 
  constant 
  during 
  these 
  stages, 
  

   and 
  moreover 
  that 
  the 
  path 
  is 
  longer 
  for 
  AB 
  than 
  for 
  BC. 
  

   That 
  this 
  is 
  actually 
  the 
  case 
  would 
  be 
  evident 
  from 
  an 
  

   inspection 
  of 
  the 
  figures 
  2, 
  3, 
  and 
  4. 
  

  

  14. 
  Curve 
  I. 
  traces 
  the 
  changes 
  in 
  the 
  case 
  in 
  which 
  

   there 
  were 
  three 
  cells 
  in 
  the 
  primary 
  circuit 
  of 
  the 
  induction- 
  

   coil, 
  and 
  Curve 
  II. 
  when 
  there 
  were 
  four. 
  It 
  is 
  evident 
  that 
  

   the 
  potential-difference 
  for 
  the 
  same 
  pressure 
  is 
  greater 
  in 
  

   the 
  second 
  case 
  than 
  in 
  the 
  first, 
  and 
  it 
  approximates 
  to 
  

   equality 
  as 
  the 
  pressure 
  decreases. 
  

  

  15. 
  Again, 
  the 
  pressure 
  at 
  which 
  the 
  band 
  appears 
  is 
  seen 
  

   to 
  be 
  higher 
  in 
  the 
  second 
  curve 
  than 
  in 
  the 
  first 
  ; 
  so 
  also 
  

   the 
  pressure 
  at 
  which 
  the 
  rotation 
  ceases, 
  the 
  difference 
  

   being 
  more 
  marked 
  in 
  the 
  first 
  case 
  than 
  in 
  the 
  second. 
  

  

  16. 
  For 
  the 
  same 
  strength 
  of 
  the 
  magnetic 
  field, 
  the 
  

   rotation 
  is 
  very 
  much 
  less 
  when 
  the 
  ring 
  is 
  the 
  cathode 
  

   than 
  when 
  the 
  disk 
  is 
  the 
  cathode. 
  

  

  It 
  is 
  found 
  that 
  there 
  is 
  no 
  essential 
  difference 
  in 
  the 
  two 
  

   cases 
  as 
  regards 
  the 
  variation 
  of 
  electric 
  intensity 
  with 
  

   pressure. 
  The 
  observed 
  difference 
  is 
  therefore 
  likely 
  to 
  be 
  

   due 
  to 
  a 
  difference 
  in 
  the 
  number 
  of 
  ions. 
  

  

  17. 
  As 
  the 
  discharge 
  from 
  an 
  induction-coil 
  is 
  inter- 
  

   mittent, 
  it 
  is 
  likely 
  that, 
  during 
  the 
  interval 
  in 
  which 
  the 
  

   discharge 
  is 
  not 
  passing, 
  a 
  certain 
  number 
  of 
  ions 
  recombine. 
  

   In 
  order 
  to 
  find 
  how 
  far 
  this 
  affects 
  the 
  rotational 
  effect, 
  a 
  

   steady 
  discharge 
  from 
  800 
  storage-cells 
  was 
  passed 
  through 
  

   a 
  tube, 
  the 
  distance 
  between 
  whose 
  electrodes 
  had 
  now 
  to 
  be 
  

   much 
  less 
  (2' 
  6 
  cm.) 
  than 
  in 
  the 
  previous 
  experiments. 
  

  

  It 
  is 
  found 
  that 
  the 
  discharge 
  passes 
  at 
  a 
  pressure 
  of 
  about 
  

   12 
  mm., 
  and 
  it 
  is, 
  as 
  is 
  to 
  be 
  expected, 
  a 
  " 
  band" 
  discharge 
  

   which 
  rotates 
  as 
  usual. 
  As 
  the 
  pressure 
  decreases 
  the 
  

   rotation 
  ultimately 
  ceases, 
  the 
  discharge 
  spreading 
  over 
  

   the 
  whole 
  ring. 
  In 
  this 
  state, 
  on 
  the 
  application 
  of 
  the 
  field 
  

   a 
  well-marked 
  spiral 
  is 
  seen 
  to 
  mark 
  the 
  path 
  of 
  the 
  ions, 
  

  

  