﻿200 
  Trowhridge 
  — 
  Investigation 
  of 
  the 
  Coherer. 
  

  

  microfarad, 
  and 
  this 
  range, 
  taken 
  together 
  with 
  that 
  of 
  the 
  

   storage 
  battery, 
  made 
  it 
  possible 
  to 
  vary 
  the 
  product 
  CY 
  = 
  Q, 
  

   within 
  a 
  comparatively 
  large 
  range, 
  the 
  upper 
  limit 
  of 
  which, 
  

   however, 
  was 
  only 
  224 
  micro-coulombs. 
  

  

  For 
  the 
  purpose 
  of 
  measuring 
  the 
  fall 
  of 
  resistance, 
  a 
  branch 
  

   circuit 
  containing 
  a 
  single 
  storage 
  cell 
  (E. 
  M. 
  F. 
  = 
  2 
  volts) 
  and 
  a 
  

   milli-ammeter 
  was 
  connected 
  to 
  the 
  ends 
  of 
  the 
  coherer. 
  The 
  

   limit 
  of 
  accuracy 
  in 
  reading 
  the 
  milli-ammeter 
  was 
  O'OOl 
  

   ampere. 
  

  

  With 
  a 
  coherer 
  of 
  the 
  type 
  described 
  by 
  Marconi, 
  I 
  found, 
  

   as 
  I 
  expected, 
  an 
  increase 
  in 
  conductivity 
  with 
  an 
  increase 
  in 
  

   quantity 
  of 
  electricity 
  sent 
  through 
  the 
  coherer 
  ; 
  however 
  on 
  

   increasing 
  Q 
  beyond 
  a 
  certain 
  point, 
  the 
  corresponding 
  increase 
  

   in 
  conductivity 
  was 
  very 
  small 
  ; 
  for 
  the 
  coherer 
  in 
  question, 
  

   for 
  example, 
  the 
  increase 
  in 
  conductivity 
  was 
  very 
  rapid 
  on 
  

   increasing 
  Q 
  up 
  to 
  about 
  15 
  micro-coulombs, 
  but 
  beyond 
  this 
  

   point, 
  doubling 
  the 
  quantity 
  only 
  produced 
  a 
  fractional 
  change 
  

   in 
  the 
  conductivity. 
  It 
  seemed 
  as 
  if 
  the 
  15 
  micro-coulombs 
  

   were 
  enougli 
  to 
  produce 
  all, 
  or 
  nearly 
  all, 
  the 
  lowering 
  possible 
  

   for 
  the 
  coherer 
  in 
  question 
  and 
  for 
  the 
  given 
  charging 
  poten- 
  

   tial 
  (a 
  point 
  I 
  will 
  explain 
  presently). 
  

  

  As 
  the 
  results 
  obtained 
  with 
  the 
  above 
  mentioned 
  coherer 
  

   were 
  not 
  concordant 
  enough 
  for 
  quantitative 
  work, 
  I 
  adopted 
  

   another 
  type 
  which 
  I 
  have 
  since 
  seen 
  described 
  in 
  a 
  paper 
  by 
  

   Branley,^ 
  in 
  which 
  he 
  claims 
  that 
  it 
  is 
  more 
  sensitive 
  than 
  the 
  

   older 
  form. 
  

  

  The 
  coherer 
  I 
  used 
  consisted 
  of 
  21 
  bicycle 
  pedal 
  balls 
  (diam- 
  

   eter 
  3"^™) 
  mounted 
  in 
  a 
  horizontal 
  glass 
  tube 
  of 
  about 
  the 
  same 
  

   bore, 
  the 
  end 
  balls 
  being 
  soldered 
  to 
  lead-wires 
  and 
  the 
  tube 
  

   being 
  provided 
  with 
  a 
  device 
  for 
  adjusting 
  the 
  pressure 
  of 
  

   contact 
  between 
  the 
  balls. 
  The 
  normal 
  resistance 
  of 
  this 
  

   coherer 
  was 
  about 
  2000 
  ohms, 
  care 
  being 
  taken 
  always 
  to 
  

   return 
  to 
  about 
  this 
  resistance 
  by 
  tapping 
  the 
  tube 
  and 
  adjust- 
  

   ing 
  the 
  pressure 
  before 
  an 
  observation 
  was 
  taken 
  on 
  the 
  fall 
  of 
  

   resistance 
  due 
  to 
  the 
  discharge 
  of 
  the 
  condenser. 
  M. 
  Branley 
  

   used 
  hard 
  steel 
  balls 
  12'"°^ 
  in 
  diameter 
  — 
  those 
  first 
  used 
  by 
  me 
  

   w^ere 
  9-5'"'" 
  in 
  diameter, 
  but 
  I 
  found 
  the 
  smaller 
  ones 
  finally 
  

   used 
  to 
  be 
  much 
  better 
  suited 
  to 
  my 
  purpose, 
  at 
  least, 
  and 
  I 
  

   should 
  judge 
  that 
  this 
  would 
  also 
  hold 
  true 
  for 
  telegraphic 
  

   purposes. 
  

  

  However, 
  I 
  made 
  no 
  attempt 
  at 
  long-distance 
  telegraphy 
  

   with 
  my 
  coherer, 
  having 
  only 
  satisfied 
  myself 
  that, 
  at 
  short 
  

   range, 
  the 
  ball 
  coherer 
  behaved 
  in 
  all 
  respects 
  like 
  a 
  filing 
  

   coherer. 
  

  

  The 
  method 
  of 
  observation 
  was 
  the 
  following 
  : 
  With 
  a 
  given 
  

   charging 
  potential 
  the 
  capacity 
  was 
  varied 
  through 
  the 
  limits 
  

  

  *Comptes 
  Rendus, 
  No. 
  18, 
  May, 
  1899. 
  

  

  