﻿Trowbridge 
  and 
  Duane 
  — 
  Velocity 
  of 
  Electric 
  Waves. 
  301 
  

  

  ondary 
  current 
  on 
  the 
  primary 
  of 
  the 
  induction 
  coil, 
  and 
  

   would 
  throw 
  a 
  greater 
  stress 
  on 
  the 
  point 
  of 
  rupture 
  of 
  the 
  

   primary 
  circuit 
  ; 
  hence 
  the 
  sparking. 
  To 
  obviate 
  the 
  difficulty 
  

   several 
  methods 
  of 
  exciting 
  the 
  induction 
  coil 
  were 
  tried, 
  with 
  

   more 
  or 
  less 
  success. 
  Finally, 
  an 
  ordinary 
  reed 
  interrupter 
  

   with 
  a 
  comparatively 
  large 
  hammer 
  and 
  anvil 
  arrangement 
  was 
  

   adopted, 
  which 
  gave 
  little 
  trouble. 
  

  

  The 
  second 
  point, 
  and 
  this 
  is 
  very 
  important 
  in 
  the 
  light 
  of 
  

   what 
  is 
  to 
  follow, 
  is 
  the 
  following 
  : 
  the 
  insertion 
  of 
  a 
  small 
  

   spark 
  gap 
  (l 
  mm 
  -3 
  mm 
  ) 
  a 
  t 
  the 
  point 
  in 
  the 
  secondary 
  circuit 
  

   marked 
  — 
  J 
  (fig. 
  1) 
  had 
  no 
  appreciable 
  effect 
  upon 
  the 
  position 
  of 
  

   the 
  nodal 
  points 
  G 
  and 
  H, 
  or 
  of 
  the 
  points 
  of 
  maximum 
  accu- 
  

   mulation 
  K 
  and 
  L. 
  The 
  form 
  of 
  the 
  wave 
  was 
  slightly 
  altered 
  

   for 
  a 
  meter 
  on 
  each 
  side 
  of 
  J, 
  and 
  the 
  bolometer 
  showed 
  a 
  

   slight 
  accumulation 
  in 
  the 
  immediate 
  neighborhood 
  of 
  the 
  spark 
  

   gap. 
  This 
  was 
  probably 
  due 
  to 
  the 
  charging 
  of 
  the 
  spark 
  

   terminals 
  to 
  a 
  sufficiently 
  high 
  potential 
  to 
  break 
  through 
  the 
  

   dielectric. 
  The 
  fact 
  that 
  the 
  insertion 
  of 
  a 
  spark 
  gap, 
  into 
  a 
  

   secondary 
  circuit 
  in 
  the 
  manner 
  described 
  has 
  no 
  effect 
  upon 
  

   the 
  length 
  of 
  the 
  waves 
  set 
  up 
  in 
  that 
  circuit 
  was 
  tested 
  for 
  a 
  

   number 
  of 
  different 
  cases, 
  (in 
  none 
  of 
  which, 
  however, 
  the 
  

   length 
  of 
  the 
  waves 
  was 
  greater 
  than 
  in 
  the 
  present 
  case) 
  and 
  

   found 
  to 
  be 
  true 
  in 
  each 
  one 
  of 
  them. 
  

  

  In 
  order 
  to 
  determine 
  the 
  time 
  of 
  vibration 
  we 
  used 
  a 
  con- 
  

   cave 
  rotating 
  mirror 
  : 
  and 
  the 
  images 
  of 
  the 
  oscillating 
  sparks 
  

   were 
  thrown 
  on 
  a 
  sensitive 
  plate. 
  If 
  the 
  mirror 
  rotated 
  about 
  

   a 
  horizontal 
  axis, 
  the 
  photographs 
  showed 
  bright 
  horizontal 
  lines 
  

   perpendicular 
  to 
  which 
  at 
  their 
  extremities 
  extended 
  two 
  series 
  

   of 
  dots. 
  The 
  distance 
  between 
  the 
  successive 
  dots 
  was 
  evidently 
  

   the 
  distance 
  on 
  the 
  plate 
  through 
  which 
  the 
  image 
  of 
  the 
  

   spark 
  gap 
  moved 
  during 
  the 
  time 
  of 
  a 
  complete 
  oscillation. 
  

   Hence 
  by 
  determining 
  the 
  speed 
  of 
  the 
  mirror 
  and 
  measur- 
  

   ing 
  the 
  distances 
  from 
  the 
  mirror 
  to 
  the 
  plate 
  the 
  time 
  of 
  

   oscillation 
  can 
  be 
  calculated. 
  

  

  The 
  advantages 
  of 
  photographing 
  the 
  secondary 
  spark 
  rather 
  

   than 
  the 
  primary 
  are 
  numerous. 
  In 
  the 
  first 
  place 
  to 
  properly 
  

   photograph 
  a 
  spark 
  it 
  is 
  necessary 
  to 
  use 
  pointed 
  terminals 
  : 
  

   but 
  experiment 
  has 
  shown 
  that 
  the 
  waves 
  excited 
  in 
  a 
  second- 
  

   ary 
  circuit 
  depend 
  to 
  a 
  large 
  extent 
  upon 
  the 
  character 
  of 
  the 
  

   primary 
  spark, 
  and 
  that 
  the 
  most 
  active 
  sparks 
  are 
  those 
  

   between 
  metallic 
  spheres 
  with 
  polished 
  surfaces. 
  It 
  is 
  true 
  

   that 
  waves 
  can 
  be 
  produced 
  by 
  sparks 
  between 
  points, 
  but 
  the 
  

   oscillations 
  are 
  not 
  so 
  powerful 
  or 
  well 
  marked. 
  In 
  the 
  second 
  

   place, 
  from 
  the 
  results 
  obtained 
  by 
  Ejerknes 
  one 
  would 
  

   expect 
  the 
  oscillations 
  in 
  the 
  secondary 
  circuit 
  to 
  be 
  much 
  

   less 
  damped 
  than 
  those 
  in 
  the 
  primary. 
  This 
  expecta- 
  

   tion 
  has 
  been 
  fully 
  realized. 
  Photographs 
  show 
  from 
  ten 
  to 
  

  

  