﻿Field 
  upon 
  the 
  Electric 
  Discharge 
  through 
  Gases. 
  265 
  

  

  correct 
  or 
  not. 
  ir 
  is 
  at 
  least 
  certain 
  that 
  a 
  magnetic 
  field 
  is 
  

   not 
  at 
  all 
  necessary 
  to 
  make 
  the 
  discharge 
  periodic. 
  

  

  Id 
  order 
  to 
  determine 
  whether 
  the 
  high 
  frequency 
  period 
  

   is 
  accompanied 
  by 
  striation 
  of 
  the 
  luminous 
  column, 
  the 
  

   following 
  tot 
  was 
  made. 
  The 
  rotating 
  mirror 
  was 
  placed 
  

   with 
  its 
  axis 
  parallel 
  to 
  the 
  tube 
  ; 
  that 
  is, 
  parallel 
  to 
  the 
  

   axis 
  of 
  the 
  column. 
  It' 
  the 
  screen 
  was 
  not 
  earthed, 
  the 
  mirror 
  

   showed 
  a 
  continuous 
  band 
  of 
  light 
  ; 
  but 
  when 
  the 
  screen 
  was 
  

   earthed, 
  this 
  image 
  was 
  at 
  once 
  replaced 
  by 
  bright 
  bands 
  

   separated 
  by 
  dark 
  spaces. 
  These 
  bands 
  were 
  parallel 
  to 
  each 
  

   other 
  and 
  to 
  the 
  column 
  of 
  light. 
  Various 
  modifications 
  of 
  

   the 
  period 
  showed 
  only 
  a 
  variation 
  in 
  the 
  width 
  of 
  the 
  dark 
  

   spaces, 
  and 
  never 
  any 
  striation 
  normal 
  to 
  the 
  axis 
  of 
  the 
  

   column. 
  It 
  follows, 
  therefore, 
  that 
  this 
  periodicity 
  is 
  due 
  

   to 
  alternate 
  formation 
  and 
  extinction 
  of 
  the 
  entire 
  luminous 
  

   column, 
  and 
  is 
  not 
  the 
  result 
  of 
  striation. 
  

  

  Professor 
  Righi's 
  conclusion, 
  that 
  a 
  magnetic 
  field 
  was 
  

   necessary 
  to 
  make 
  the 
  discharge 
  periodic, 
  w 
  r 
  as 
  natural, 
  as 
  he 
  

   used 
  a 
  high 
  potential 
  cathode 
  ; 
  with 
  this 
  arrangement, 
  no 
  

   electrons 
  are 
  given 
  off 
  until 
  the 
  potential 
  is 
  sufficient 
  to 
  

   produce 
  ionization 
  of 
  the 
  gas, 
  and 
  when 
  this 
  occurs 
  the 
  

   current 
  is 
  continuous. 
  It 
  is, 
  however, 
  maintained 
  by 
  some 
  

   that, 
  even 
  in 
  the 
  ordinary 
  Crookes's 
  tube, 
  the 
  current 
  is 
  

   periodic 
  as 
  shown 
  by 
  its 
  striated 
  appearance 
  ; 
  the 
  evidence 
  

   is 
  rather 
  against 
  this 
  opinion. 
  But 
  when 
  a 
  magnetic 
  field 
  is 
  

   applied 
  which 
  lowers 
  the 
  potential 
  fall 
  at 
  the 
  cathode 
  and 
  

   impels 
  electrons 
  away 
  from 
  the 
  cathode, 
  it 
  is 
  reasonable 
  to 
  

   expect 
  that 
  so 
  many 
  electrons 
  may 
  be 
  driven 
  away 
  as 
  to 
  stop 
  

   the 
  current 
  until 
  they 
  again 
  accumulate. 
  At 
  any 
  rate, 
  it 
  is 
  

   evident 
  from 
  our 
  experiments 
  that 
  the 
  discharge 
  from 
  a 
  

   AVehnelt 
  cathode 
  is 
  periodic 
  if 
  the 
  tube 
  be 
  covered 
  with 
  a 
  

   metal 
  screen 
  connected 
  to 
  earth, 
  whether 
  there 
  is 
  a 
  magnetic 
  

   field 
  acting 
  or 
  not. 
  

  

  The 
  evidence 
  is 
  conflicting 
  as 
  to 
  whether 
  the 
  magnetic 
  

   ray- 
  (that 
  is, 
  the 
  portion 
  of 
  the 
  discharge 
  which 
  bends 
  toward 
  

   or 
  away 
  from 
  the 
  pole 
  of 
  an 
  auxiliary 
  magnet) 
  are 
  streams 
  

   of 
  electrically 
  neutral 
  or 
  negatively 
  charged 
  particles. 
  Righi 
  

   deflected 
  the 
  magnetic 
  rays 
  into 
  a 
  branch 
  tube, 
  and 
  states 
  

   that 
  no 
  charge 
  was 
  given 
  by 
  them 
  to 
  a 
  Faraday 
  pail. 
  Bui 
  

   Thirkill 
  * 
  was 
  able 
  to 
  coil 
  them 
  into 
  a 
  very 
  fine 
  spiral 
  along 
  

   the 
  line- 
  of 
  magnetic 
  force 
  passing 
  through 
  the 
  cathode. 
  He 
  

   also 
  collected 
  them 
  in 
  a 
  Faraday 
  pail 
  and 
  found 
  they 
  carried 
  

   a 
  negative 
  charge. 
  Both 
  of 
  Thirkili's 
  results 
  have 
  been 
  

  

  * 
  Thirkill, 
  Proc. 
  Roy. 
  Hoc. 
  vol. 
  lxxxiii. 
  p. 
  324 
  (1910). 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  26. 
  No. 
  152. 
  Aug. 
  1913. 
  T 
  

  

  