﻿G. 
  Y. 
  Maclean 
  — 
  Velocity 
  of 
  Electric 
  Waves 
  in 
  Air. 
  5 
  

  

  the 
  ordinary 
  metal 
  filings 
  coherer. 
  The 
  platinum 
  globules 
  

   are 
  brought 
  into 
  slightest 
  contact, 
  the 
  passage 
  of 
  an 
  electric 
  

   wave 
  causes 
  the 
  globules 
  to 
  cohere 
  more 
  closely 
  ; 
  thus 
  the 
  resist- 
  

   ance 
  to 
  the 
  batter}^ 
  current 
  through 
  the 
  coherer 
  is 
  lessened, 
  as 
  

   is 
  shown 
  by 
  the 
  increased 
  deflection 
  of 
  the 
  needle. 
  The 
  coher- 
  

   ing 
  of 
  the 
  metal 
  filings 
  (in 
  this 
  case 
  the 
  two 
  globules) 
  seems 
  

   to 
  be 
  due 
  to 
  an 
  electrostatic 
  effect 
  produced 
  upon 
  them 
  by 
  

   the 
  passage 
  over 
  them 
  of 
  the 
  electro-magnetic 
  waves. 
  Further 
  

   evidence 
  of 
  this 
  will 
  be 
  given 
  later. 
  

  

  Before 
  concluding 
  the 
  description 
  of 
  the 
  coherer, 
  it 
  will 
  be 
  

   of 
  advantage 
  to 
  give 
  an 
  account 
  of 
  the 
  oscillator 
  circuit 
  and 
  

   its 
  accessories. 
  

  

  The 
  electric 
  spark 
  which 
  sent 
  forth 
  the 
  trains 
  of 
  electro- 
  

   magnetic 
  waves 
  through 
  the 
  free 
  air 
  between 
  the 
  oscillator 
  and 
  

   coherer, 
  was 
  given 
  out 
  by 
  a 
  large 
  Ruhmkorff 
  coil 
  in 
  connection 
  

   with 
  a 
  storage 
  battery 
  of 
  27 
  cells 
  in 
  parallel 
  with 
  a 
  voltage 
  of 
  

   about 
  55, 
  

  

  The 
  oscillator 
  consisted 
  of 
  two 
  platinum 
  globules 
  1 
  mm. 
  in 
  

   diameter 
  attached 
  to 
  the 
  ends 
  of 
  a 
  platinum 
  spiral 
  making- 
  

   two 
  convolutions 
  about 
  copper 
  terminals, 
  exactly 
  like 
  those 
  in 
  

   the 
  coherer. 
  This 
  oscillator 
  was 
  placed 
  in 
  the 
  secondary 
  spark 
  

   gap 
  of 
  the 
  Kuhmkorff 
  coil. 
  The 
  oscillator 
  globules 
  were 
  always 
  

   kept 
  4 
  mms. 
  apart 
  from 
  each 
  other. 
  The 
  same 
  kind 
  and 
  the 
  

   same 
  amount 
  of 
  capacity 
  and 
  self-induction 
  were 
  used 
  as 
  in 
  the 
  

   coherer 
  circuit. 
  The 
  coherer 
  and 
  oscillator 
  circuits 
  were 
  thus 
  

   tuned 
  to 
  one 
  another. 
  

  

  The 
  primary 
  spark-gap 
  of 
  the 
  Ruhmkorff 
  coil 
  was 
  removed 
  

   to 
  a 
  distant 
  mercury 
  break. 
  The 
  mercury 
  break 
  was 
  of 
  special 
  

   construction. 
  Three 
  storage 
  cells 
  drove 
  a 
  motor, 
  which 
  in 
  turn 
  

   caused 
  a 
  plunger 
  to 
  play 
  in 
  and 
  out 
  of 
  a 
  mercury 
  glass 
  cup. 
  

   On 
  opposite 
  sides 
  of 
  the 
  glass 
  cup 
  were 
  secured 
  a 
  glass 
  tube 
  

   shoulder, 
  1 
  mm. 
  or 
  so 
  above 
  the 
  level 
  of 
  the 
  mercury. 
  To 
  

   tliese 
  glass 
  shoulders 
  were 
  attached 
  rubber 
  tubes, 
  one 
  leading 
  

   from 
  a 
  water 
  tank 
  and 
  the 
  other 
  to 
  a 
  sink. 
  Water 
  was 
  

   siphoned 
  from 
  the 
  tank 
  through 
  the 
  glass 
  cup, 
  over 
  the 
  sur- 
  

   face 
  of 
  the 
  mercur3^ 
  Thus 
  the 
  surface 
  of 
  the 
  mercury 
  was 
  

   always 
  clean. 
  A 
  pinch-cock 
  was 
  flxed 
  to 
  the 
  first 
  rubber 
  tube 
  

   so 
  that 
  the 
  strength 
  of 
  the 
  stream 
  of 
  the 
  running 
  water 
  could 
  

   be 
  regulated 
  and 
  kept 
  constant. 
  When 
  the 
  stream 
  of 
  water 
  

   was 
  properly 
  regulated, 
  sparking 
  at 
  the 
  oscillator 
  could 
  be 
  con- 
  

   tinued 
  for 
  hours 
  at 
  a 
  time, 
  the 
  sparks 
  always 
  being 
  perfectly 
  

   regular 
  and 
  uniform. 
  Before 
  the 
  addition 
  of 
  the 
  stream 
  of 
  

   water 
  over 
  the 
  surface 
  of 
  the 
  mercury, 
  the 
  break 
  gave 
  end- 
  

   less 
  trouble, 
  needing 
  attention 
  almost 
  every 
  two 
  or 
  three 
  

   minutes, 
  but 
  under 
  the 
  conditions 
  adopted 
  it 
  required 
  very 
  lit- 
  

   tle 
  attention. 
  However, 
  care 
  had 
  to 
  be 
  taken 
  to 
  prevent 
  a 
  too 
  

   free 
  flowing 
  of 
  the 
  water, 
  for 
  otherwise 
  the 
  nature 
  of 
  the 
  

  

  