﻿Field 
  upon 
  the 
  Electric 
  Discharge 
  through 
  Gases. 
  2G3 
  

  

  And 
  still 
  more 
  important 
  is 
  his 
  statement 
  that 
  a 
  mngnetic 
  

   field 
  is 
  necessary 
  for 
  a 
  periodic 
  discbarge 
  and 
  that 
  his 
  theory 
  

   alone 
  will 
  account 
  for 
  this 
  periodicity. 
  There 
  is 
  no 
  doubt 
  

   that 
  the 
  discharge 
  is 
  periodic 
  when 
  it 
  is 
  acted 
  upon 
  by 
  a 
  

   magnetic 
  field, 
  because 
  a 
  high 
  musical 
  note 
  is 
  heard 
  and 
  a 
  

   telephone 
  placed 
  in 
  the 
  circuit 
  buzzes, 
  but 
  the 
  action 
  

   requires 
  a 
  more 
  careful 
  inquiry. 
  

  

  For 
  this 
  purpose 
  a 
  four 
  sided 
  rotating 
  mirror 
  was 
  set 
  up 
  

   with 
  its 
  axis 
  vertical 
  ; 
  that 
  is, 
  perpendicular 
  to 
  the 
  axis 
  of 
  

   the 
  tube. 
  It 
  \ias 
  driven 
  by 
  an 
  electric 
  motor, 
  controlled 
  to 
  

   give 
  any 
  speed 
  up 
  to 
  1800 
  revolutions 
  per 
  minute. 
  To 
  

   examine 
  the 
  appearance 
  of 
  the 
  discharge 
  in 
  different 
  parts 
  

   of 
  the 
  tube, 
  a 
  screen 
  with 
  an 
  opening 
  about 
  two 
  centimetres 
  

   square 
  was 
  placed 
  so 
  as 
  to 
  intercept 
  all 
  the 
  light 
  except 
  that 
  

   from 
  the 
  desired 
  place. 
  When 
  the 
  discharge 
  was 
  a 
  con- 
  

   tinuous 
  one, 
  the 
  light 
  reflected 
  from 
  the 
  mirror 
  was 
  broadened 
  

   into 
  a 
  uniform 
  band. 
  On 
  the 
  other 
  hand, 
  an 
  intermittent 
  

   discharge 
  caused 
  the 
  band 
  of 
  light 
  io 
  be 
  crossed 
  by 
  vertical 
  

   dark 
  bands. 
  These 
  dark 
  bands 
  moved 
  either 
  to 
  the 
  left 
  

   or 
  to 
  the 
  right 
  according 
  as 
  the 
  speed 
  of 
  the 
  mirror 
  was 
  too 
  

   slow 
  or 
  too 
  fast. 
  In 
  all 
  cases 
  the 
  mirror 
  could 
  be 
  driven 
  

   fast 
  enough 
  to 
  reverse 
  the 
  motion 
  of 
  the 
  bands. 
  For 
  short 
  

   intervals, 
  the 
  motor 
  was 
  regulated 
  to 
  keep 
  the 
  bands 
  

   stationary 
  in 
  order 
  to 
  obtain 
  an 
  approximate 
  determination 
  

   of 
  the 
  periodicity. 
  

  

  The 
  results 
  of 
  the 
  periodicity 
  are 
  given 
  in 
  the 
  table 
  : 
  — 
  - 
  

  

  Screen 
  insulated 
  ; 
  cathode 
  insulated 
  ; 
  no 
  magnetic 
  field 
  : 
  dis- 
  

   charge 
  is 
  continuous. 
  

  

  Screen 
  earthed; 
  cathode 
  insulated 
  ; 
  no 
  magnetic 
  field 
  : 
  discharge 
  

   is 
  periodic. 
  

  

  Screen 
  earthed 
  ; 
  cathode 
  earthed 
  ; 
  no 
  magnetic 
  field 
  : 
  discharge 
  

   is 
  periodic. 
  

  

  Screen 
  earthed; 
  cathode 
  earthed; 
  magnetic 
  field 
  acting: 
  dis- 
  

   charge 
  is 
  periodic. 
  

  

  If 
  we 
  call 
  the 
  period 
  normal 
  when 
  the 
  screen 
  alone 
  is 
  

   earthed 
  and 
  the 
  magnetic 
  field 
  is 
  not 
  acting, 
  then, 
  if 
  we 
  also 
  

   earth 
  the 
  cathode, 
  the 
  period 
  is 
  very 
  noticeably 
  lengthened. 
  

   If. 
  instead 
  of 
  earthing 
  the 
  cathode, 
  the 
  magnetic 
  field 
  is 
  

   applied, 
  the 
  period 
  becomes 
  shorter 
  than 
  the 
  normal 
  ; 
  but 
  

   the 
  change 
  is 
  not 
  so 
  great 
  as 
  when 
  the 
  cathode 
  was 
  earthed. 
  

   At 
  times, 
  when 
  the 
  cathode 
  was 
  earthed 
  and 
  the 
  magnetic 
  

   fir-Id 
  also 
  acting, 
  two 
  notes 
  could 
  be 
  distinguished 
  ; 
  the 
  low 
  

   note 
  due 
  to 
  the 
  former 
  and 
  the 
  high 
  one 
  produced 
  by 
  the 
  

   magnetic 
  field. 
  In 
  the 
  mirror, 
  the 
  light 
  was 
  broken 
  up 
  into 
  

  

  