﻿4 
  G. 
  V. 
  Maclean 
  — 
  Velocity 
  of 
  Electric 
  Waves 
  in 
  Air. 
  

  

  Pavia 
  battery 
  of 
  peculiar 
  construction 
  giving 
  a 
  steady 
  current, 
  

   a 
  resistance 
  box 
  in 
  which 
  a 
  resistance 
  of 
  over 
  100 
  ohms 
  is 
  

   always 
  kept, 
  and 
  a 
  Direct 
  Reading 
  Mille-Anipere 
  meter. 
  The 
  

   current 
  is 
  shnnted 
  by 
  a 
  suitable 
  resistance 
  before 
  passing 
  

   through 
  the 
  coherer. 
  

  

  The 
  capacity 
  and 
  self 
  induction 
  of 
  the 
  coherer 
  are 
  respec- 
  

   tively 
  two 
  sheets 
  of 
  tin 
  foil 
  14*5 
  cm. 
  by 
  14'1 
  cms. 
  shellacked 
  

   to 
  the 
  sides 
  of 
  a 
  glass 
  plate 
  30"4 
  cms. 
  long, 
  30-4 
  cms. 
  wide 
  and 
  

  

  5 
  cms. 
  thick, 
  and 
  two 
  copper 
  wires 
  9*62 
  cms. 
  long, 
  and 
  '3 
  mm. 
  

   in 
  diameter, 
  parallel 
  to 
  each 
  other 
  and 
  distant 
  5 
  cms. 
  These 
  

   two 
  wires 
  had 
  two 
  of 
  their 
  ends 
  soldered 
  to 
  two 
  strips 
  of 
  

   brush 
  copper 
  which 
  latter 
  made 
  close 
  contact 
  with 
  the 
  tin 
  foil, 
  

   by 
  means 
  of 
  hard 
  wax 
  ; 
  the 
  other 
  ends 
  passed 
  into 
  two 
  small 
  

   binding 
  posts 
  on 
  the 
  caps 
  of 
  the 
  coherer. 
  The 
  current 
  through 
  

   the 
  coherer 
  is 
  governed 
  by 
  a 
  key. 
  This 
  constituted 
  the 
  coherer 
  

   or 
  receiving 
  circuit. 
  AH 
  the 
  wires 
  used 
  throughout 
  the 
  whole 
  

   experiment 
  were 
  well 
  insulated, 
  twisted 
  and 
  kept 
  as 
  far 
  as 
  pos- 
  

   sible 
  out 
  of 
  the 
  direct 
  course 
  of 
  the 
  electric 
  waves, 
  so 
  that 
  any 
  

   influence 
  the 
  wires 
  might 
  exert 
  upon 
  the 
  results 
  was 
  reduced 
  

   to 
  a 
  minimum. 
  

  

  The 
  different 
  parts 
  of 
  the 
  coherer 
  circuit 
  can 
  be 
  seen 
  in 
  the 
  

   photograph, 
  Plate 
  I. 
  

  

  The 
  platinum 
  globules 
  w^ere 
  first 
  brought 
  into 
  the 
  slightest 
  

   possible 
  contact 
  by 
  carefully 
  adjusting 
  a 
  micrometer 
  screw. 
  

   When 
  such 
  a 
  contact 
  had 
  been 
  effected 
  was 
  known 
  by 
  watching 
  

   the 
  motion 
  of 
  the 
  Mille-Ampere 
  meter 
  needle. 
  Upon 
  causing 
  

   a 
  train 
  of 
  electric 
  waves 
  to 
  pass 
  the 
  coherer, 
  the 
  resistance 
  of 
  

   the 
  latter 
  is 
  lessened 
  by 
  the 
  globules 
  moving 
  into 
  closer 
  con- 
  

   tact, 
  that 
  is 
  to 
  say 
  the 
  globules 
  are 
  made 
  to 
  cohere. 
  This 
  aug- 
  

   mentation 
  of 
  the 
  globules 
  is 
  instantly 
  evidenced 
  by 
  the 
  

   increased 
  throw 
  of 
  the 
  Mille-Ampere 
  meter 
  needle. 
  

  

  The 
  coherer 
  here 
  employed 
  is 
  the 
  outcome 
  of 
  a 
  great 
  many 
  

   experiments 
  with 
  Lodge 
  coherers, 
  Marconi 
  coherers 
  or 
  Branly 
  

   tubes. 
  Tubes 
  containing 
  metal 
  filings 
  of 
  all 
  kinds 
  and 
  of 
  

   different 
  degrees 
  of 
  fineness, 
  as 
  iron, 
  silver, 
  platinum, 
  copper, 
  

   zinc, 
  nickel, 
  magnesium, 
  brass, 
  granulated 
  arc 
  carbon 
  with 
  

   copper, 
  or 
  brass, 
  or 
  iron 
  terminals, 
  either 
  magnetized 
  or 
  non- 
  

   magnetized, 
  all 
  proved 
  to 
  be 
  unsuitable 
  for 
  measurement 
  of 
  the 
  

   electric 
  wave-lengths, 
  in 
  not 
  possessing 
  the 
  proper 
  degree 
  of 
  

   sensitiveness. 
  They 
  all 
  had 
  the 
  common 
  fault 
  of 
  not 
  allowing 
  

   the 
  Mille-Ampere 
  meter 
  needle 
  to 
  return 
  quickly 
  to 
  its 
  zero 
  

   reading. 
  Decohering 
  of 
  the 
  coherer 
  in 
  the 
  present 
  form 
  is 
  

   accomplished 
  almost 
  instantly 
  by 
  the 
  elasticity 
  of 
  the 
  plati- 
  

   num 
  spirals, 
  upon 
  opening 
  the 
  circuit 
  key. 
  No 
  tapping 
  is 
  

   required 
  as 
  in 
  other 
  kinds 
  of 
  coherers. 
  In 
  fact 
  this 
  form 
  of 
  a 
  

   coherer 
  alone 
  of 
  all 
  tried 
  gave 
  good 
  working 
  results. 
  More- 
  

   over, 
  the 
  above 
  coherer 
  at 
  once 
  shows 
  the 
  manner 
  of 
  action 
  of 
  

  

  