﻿810 
  Mr. 
  H. 
  L. 
  P. 
  Jolly 
  on 
  the 
  

  

  wire 
  stretched 
  across 
  it. 
  the 
  platinum 
  wire 
  carrying 
  the 
  

   discharge-tube 
  current. 
  But 
  when 
  used 
  in 
  connexion 
  with 
  a 
  

   Thomson 
  astatic 
  galvanometer 
  this 
  arrangement 
  did 
  not 
  

   prove 
  sufficiently 
  sensitive. 
  

  

  A 
  method 
  which 
  gave 
  better 
  results 
  was 
  to 
  measure 
  the 
  

   mean 
  potential 
  difference 
  between 
  the 
  ends 
  of 
  a 
  known 
  

   (non-inductive) 
  resistance 
  in 
  series 
  with 
  the 
  discharge-tube. 
  

  

  This 
  potential 
  difference 
  was 
  measured 
  at 
  first 
  by 
  a 
  gold-leaf 
  

   electrometer, 
  and 
  some 
  consistent 
  readings 
  were 
  obtained. 
  

   But 
  the 
  necessity 
  of 
  calibrating 
  the 
  gold-leaf 
  after 
  each 
  set 
  

   of 
  observations 
  rendered 
  the 
  method 
  very 
  tedious. 
  

  

  A 
  curve 
  obtained 
  in 
  this 
  way, 
  showing 
  the 
  variation 
  of 
  

   current 
  when 
  the 
  source 
  of 
  E.M.F. 
  was 
  kept 
  constant 
  and 
  

   the 
  pressure 
  allowed 
  to 
  vary 
  over 
  wide 
  limits, 
  is 
  appended 
  

   (curve 
  3, 
  PL 
  XIV.). 
  

  

  The 
  less 
  steep 
  curve 
  represents 
  the 
  same 
  observations 
  

   plotted 
  with 
  the 
  abscissae 
  on 
  a 
  scale 
  10 
  times 
  as 
  large, 
  so 
  as 
  

   to 
  show 
  the 
  behaviour 
  of 
  the 
  curve 
  at 
  lower 
  pressures. 
  

  

  Obviously, 
  from 
  the 
  regularity 
  of 
  the 
  curve, 
  this 
  method 
  

   of 
  measuring 
  small 
  alternating 
  currents 
  is 
  a 
  practicable 
  one. 
  

  

  The 
  mean 
  potential 
  difference 
  between 
  the 
  ends 
  of 
  a 
  known 
  

   resistance 
  was 
  later 
  measured 
  by 
  an 
  alternating 
  voltmeter 
  

   specially 
  made 
  for 
  a 
  frequency 
  of 
  90 
  per 
  second. 
  

  

  Finally, 
  in 
  connexion 
  with 
  the 
  measurement 
  of 
  the 
  currents, 
  

   experiments 
  were 
  made 
  in 
  which 
  a 
  discharge-tube 
  of 
  the 
  

   form 
  shown 
  in 
  fig. 
  5 
  (PL 
  XIII.) 
  was 
  placed 
  in 
  series 
  with 
  

   the 
  tube 
  used 
  as 
  source 
  of 
  radiation. 
  Each 
  electrode 
  consists 
  

   of 
  a 
  long 
  brass 
  rod 
  about 
  3 
  mm. 
  in 
  diameter, 
  and 
  the 
  tw 
  r 
  o 
  are 
  

   placed 
  as 
  nearly 
  as 
  possible 
  co-axially. 
  According 
  to 
  H. 
  A. 
  

   Wilson's 
  experiments, 
  the 
  length 
  of 
  the 
  glow 
  on 
  the 
  cathode 
  

   is 
  nearly 
  proportional 
  to 
  the 
  current 
  and 
  inversely 
  pro- 
  

   portional 
  to 
  the 
  pressure. 
  

  

  We 
  have, 
  therefore, 
  a 
  method 
  of 
  determining 
  not 
  only 
  the 
  

   maximum 
  current 
  but 
  also 
  the 
  variation 
  of 
  its 
  value 
  over 
  

   the 
  natural 
  cycle 
  of 
  1/90 
  sec, 
  for 
  by 
  examining 
  the 
  tube 
  in 
  

   a 
  rotating 
  mirror 
  we 
  may 
  draw 
  out 
  its 
  image 
  into 
  a 
  shape 
  

   like 
  that 
  showm 
  above, 
  clearly 
  indicating 
  the 
  sinusoidal 
  wave- 
  

   form 
  of 
  a 
  purely 
  alternating 
  current. 
  

  

  When 
  a 
  condenser 
  discharge 
  is 
  sent 
  through 
  such 
  a 
  tube, 
  

   interesting 
  variations 
  of 
  the 
  above 
  curve 
  are 
  obtained. 
  The 
  

   glow 
  on 
  the 
  cathode 
  does 
  not 
  ahvays 
  extend 
  to 
  a 
  uniform 
  

   length 
  on 
  all 
  sides 
  of 
  the 
  rod, 
  and 
  may 
  make 
  a 
  measurement 
  

   of 
  its 
  mean 
  length 
  difficult. 
  Further, 
  it 
  is 
  doubtful 
  whether 
  

   the 
  length 
  is 
  proportional 
  to 
  the 
  current 
  in 
  the 
  case 
  of 
  the 
  

   condenser 
  discharges, 
  where 
  we 
  have 
  a 
  very 
  large 
  current 
  

   passing 
  for 
  a 
  very 
  short 
  time 
  a 
  certain 
  number 
  of 
  times 
  per 
  

  

  