﻿[ 
  718 
  j 
  

  

  LX. 
  A 
  Vacuum- 
  Tube 
  Model 
  for 
  Demonstrating 
  the 
  Propagation 
  

   of 
  Alternate 
  Currents 
  in 
  Cables. 
  By 
  Dr. 
  C. 
  V. 
  Drysdale 
  *. 
  

  

  [Plates 
  XXII. 
  & 
  XXIII.] 
  

  

  AS 
  is 
  well 
  known, 
  the 
  propagation 
  of 
  electric 
  waves 
  along 
  

   conductors 
  can 
  be 
  conveniently 
  shown 
  on 
  the 
  small 
  

   scale 
  by 
  means 
  of 
  a 
  helix, 
  fed 
  from 
  a 
  suitable 
  source 
  of 
  high 
  

   frequency, 
  high 
  potential 
  oscillations. 
  In 
  this 
  case 
  the 
  helix 
  

   is 
  found 
  to 
  glow 
  with 
  brush 
  discharges 
  at 
  the 
  antinodes 
  of 
  

   potential, 
  remaining 
  dark 
  at 
  the 
  nodes. 
  Experiments 
  of 
  

   this 
  kind 
  have 
  been 
  shown 
  by 
  Prof. 
  J. 
  A. 
  Fleming 
  and 
  by 
  

   G. 
  Seibtf. 
  The 
  glow 
  is, 
  however, 
  very 
  faint, 
  and 
  may 
  be 
  

   rendered 
  much 
  more 
  visible 
  by 
  the 
  use 
  of 
  a 
  vacuum-tube, 
  

   as 
  Prof. 
  Fleming 
  himself 
  suggests. 
  

  

  When 
  experimenting 
  lately 
  in 
  this 
  direction, 
  the 
  present 
  

   writer 
  noticed 
  that 
  if 
  a 
  long 
  vacuum-tube 
  was 
  laid 
  parallel 
  to 
  

   the 
  helix, 
  light 
  and 
  dark 
  portions 
  were 
  observed, 
  but 
  that 
  

   these 
  did 
  not 
  coincide 
  with 
  the 
  points 
  of 
  maximum 
  and 
  

   minimum 
  potential 
  respectively; 
  and 
  it 
  was 
  of 
  course 
  evident 
  

   that 
  such 
  a 
  tube 
  is 
  influenced 
  not 
  by 
  the 
  actual 
  potential, 
  but 
  

  

  bv 
  its 
  slope 
  or 
  ^— 
  along 
  the 
  tube. 
  As 
  the 
  intrinsic 
  equation 
  

  

  . 
  <** 
  av 
  . 
  . 
  

  

  of 
  propagation 
  is 
  ^— 
  =IC, 
  it 
  is 
  clear 
  that 
  the 
  brightness 
  

  

  of 
  the 
  tube 
  is 
  proportional 
  to 
  the 
  current 
  in 
  the 
  helix, 
  and 
  

   that 
  therefore 
  the 
  whole 
  distribution 
  of 
  current 
  and 
  potential 
  

   can 
  be 
  exhibited 
  by 
  vacuum-tubes 
  fixed 
  parallel 
  to 
  and 
  per- 
  

   pendicular 
  to 
  the 
  helix 
  respectively. 
  A 
  few 
  photographs 
  of 
  

   the 
  effects 
  observed 
  were 
  published 
  in 
  a 
  recent 
  paper 
  J, 
  but 
  

   these 
  were 
  obtained 
  by 
  successive 
  exposure 
  of 
  the 
  same 
  tube 
  

   in 
  different 
  positions. 
  

  

  In 
  order 
  to 
  show 
  these 
  effects 
  with 
  the 
  greatest 
  convenience 
  

   the 
  model 
  shown 
  in 
  fig. 
  1 
  (PL 
  XXII.) 
  has 
  been 
  made. 
  The 
  

   helix 
  is 
  182 
  cms. 
  long 
  and 
  2*65 
  cms. 
  in 
  external 
  diameter. 
  It 
  

   is 
  wound 
  on 
  a 
  glass 
  tube 
  with 
  25*5 
  turns 
  per 
  cm. 
  of 
  13 
  mil. 
  

   D.S.C. 
  wire; 
  and 
  has 
  a 
  resistance 
  of 
  *43 
  6), 
  an 
  inductance 
  of 
  

  

  TT 
  2 
  d 
  2 
  n 
  2 
  

  

  irtnn 
  = 
  44 
  microhenrys 
  per 
  cm. 
  The 
  capacity 
  from 
  expe- 
  

   riments 
  with 
  a 
  metal 
  tube 
  of 
  similar 
  diameter 
  appears 
  to 
  be 
  

   9*2 
  x 
  10~ 
  6 
  mfd. 
  per 
  cm. 
  

  

  * 
  Communicated 
  by 
  the 
  Physical 
  Society 
  : 
  read 
  March 
  27, 
  1908. 
  

  

  f 
  See 
  ' 
  The 
  Principles 
  of 
  Electric 
  Wave 
  Telegraphy,' 
  by 
  Dr. 
  J. 
  A. 
  

   Fleming, 
  pp. 
  251-261. 
  

  

  X 
  " 
  The 
  Theory 
  of 
  Alternate 
  Current 
  Transmission 
  in 
  Cables/' 
  Elec- 
  

   trician, 
  vol. 
  

  

  