﻿302 
  Trowbridge 
  and 
  Duane 
  — 
  Velocity 
  of 
  Electric 
  Waves. 
  

  

  twelve 
  times 
  as 
  many 
  oscillations 
  in 
  the 
  secondary 
  as 
  in 
  the 
  

   primary. 
  In 
  the 
  third 
  and 
  by 
  no 
  means 
  the 
  least 
  important 
  

   case, 
  the 
  question 
  how 
  close 
  the 
  resonance 
  is 
  does 
  not 
  affect 
  

   the 
  accuracy 
  of 
  the 
  results. 
  By 
  photographing 
  the 
  sparks 
  

   in 
  the 
  secondary 
  the 
  period 
  of 
  oscillation 
  is 
  determined, 
  

   not 
  of 
  a 
  circuit 
  that 
  is 
  altered 
  until 
  by 
  trial 
  it 
  is 
  found 
  to 
  have 
  

   as 
  nearly 
  as 
  possible 
  the 
  same 
  period 
  of 
  vibration 
  as 
  the 
  circuit 
  

   on 
  which 
  the 
  length 
  of 
  the 
  wave 
  is 
  measured, 
  bat 
  that 
  of 
  the 
  

   circuit 
  along 
  which 
  the 
  wave 
  itself 
  is 
  actually 
  traveling. 
  

  

  The 
  great 
  difficulty 
  to 
  be 
  overcome 
  is 
  the 
  production 
  of 
  

   secondary 
  oscillations 
  that 
  will 
  produce 
  sparks 
  of 
  sufficient 
  

   brightness 
  to 
  photograph. 
  It 
  is 
  comparatively 
  an 
  easy 
  task 
  to 
  

   photograph 
  the 
  primary 
  spark, 
  but 
  in 
  order 
  to 
  photograph 
  the 
  

   secondary 
  the 
  dimensions 
  of 
  the 
  circuit 
  must 
  be 
  chosen 
  with 
  

   great 
  care. 
  

  

  With 
  a 
  view 
  to 
  increasing 
  the 
  light 
  of 
  the 
  spark 
  together 
  with 
  

   the 
  length 
  of 
  the 
  waves, 
  it 
  seemed 
  desirable 
  to 
  lengthen 
  the 
  

   period 
  of 
  oscillation 
  by 
  enlarging 
  the 
  condensers 
  rather 
  than 
  

   by 
  increasing 
  the 
  self 
  induction 
  of 
  the 
  primary 
  circuit. 
  A 
  

   castor 
  oil 
  condenser 
  was 
  therefore 
  designed 
  and 
  constructed 
  

   on 
  the 
  following 
  plan. 
  Eight 
  plates 
  (25 
  cm 
  X 
  20 
  cm 
  ) 
  were 
  cut 
  

   out 
  of 
  sheet 
  zinc, 
  and 
  were 
  held 
  in 
  vertical 
  planes 
  side 
  by 
  side 
  

   2 
  cm 
  apart 
  by 
  a 
  suitable 
  hard 
  rubber 
  frame. 
  The 
  plates 
  were 
  

   entirely 
  immersed 
  in 
  castor 
  oil 
  contained 
  in 
  a 
  glass 
  jar. 
  They 
  

   were 
  connected 
  together 
  in 
  the 
  manner 
  shown 
  in 
  fig. 
  2. 
  

  

  The 
  plates 
  marked 
  a, 
  c 
  and 
  e 
  were 
  fastened 
  to 
  the 
  con- 
  

   ductor 
  A 
  B 
  and 
  formed 
  one 
  armature 
  of 
  the 
  condenser. 
  

   Those 
  marked 
  d, 
  f 
  and 
  h 
  were 
  joined 
  to 
  C 
  D 
  and 
  formed 
  the 
  

   other 
  armature. 
  The 
  two 
  ends 
  of 
  the 
  secondary 
  circuit 
  E, 
  G, 
  

   J, 
  H, 
  F, 
  were 
  fastened 
  to 
  the 
  plate 
  b 
  and 
  g. 
  The 
  plane 
  of 
  

   the 
  secondary 
  circuit 
  was 
  50 
  cm 
  , 
  and 
  that 
  of 
  the 
  primary 
  3 
  cm 
  

   above 
  the 
  upper 
  edge 
  of 
  the 
  condenser 
  plate. 
  The 
  total 
  

   length 
  of 
  the 
  secondary 
  circuit 
  from 
  one 
  condenser 
  plate 
  

   through 
  E, 
  G, 
  J, 
  H, 
  F, 
  to 
  the 
  other 
  plate 
  was 
  6338 
  cm 
  The 
  

   circuit 
  consisted 
  of 
  copper 
  wire 
  (diameter 
  -215 
  cm 
  ) 
  supported 
  at 
  

   each 
  end 
  by 
  suitable 
  wooden 
  frames, 
  and 
  also 
  once 
  in 
  the 
  

   middle 
  by 
  hard 
  rubber 
  hooks 
  fastened 
  by 
  long 
  pieces 
  of 
  twine 
  

   to 
  a 
  wooden 
  crossbar 
  above. 
  The 
  distances 
  from 
  F 
  to 
  E 
  and 
  

   from 
  K 
  to 
  L 
  were 
  30 
  cm 
  and 
  a 
  spark 
  gap 
  with 
  pointed 
  tin 
  ter- 
  

   minals 
  was 
  inserted 
  at 
  J. 
  The 
  primary 
  circuit 
  consisted 
  of 
  

   copper 
  wire 
  (diameter 
  *34 
  cm 
  ). 
  The 
  distances 
  between 
  the 
  two 
  

   parts 
  A 
  B 
  and 
  CD 
  was 
  45 
  cm 
  . 
  The 
  portion 
  B 
  D 
  contained 
  a 
  

   spark 
  gap 
  with 
  platinum-faced 
  spherical 
  terminals, 
  and 
  was 
  made 
  

   so 
  as 
  to 
  slide 
  back 
  and 
  forth, 
  to 
  and 
  from 
  the 
  condenser. 
  The 
  

   motion 
  of 
  this 
  movable 
  piece 
  varied 
  the 
  self 
  induction 
  and 
  

   therefore 
  the 
  period 
  of 
  oscillation 
  of 
  the 
  primary 
  circuit. 
  By 
  

   this 
  means 
  the 
  circuits 
  were 
  brought 
  into 
  resonance. 
  With 
  

  

  