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  131 
  j 
  

  

  XIII. 
  On 
  the 
  Relation 
  of 
  the 
  Audibility 
  Factor 
  of 
  a 
  Shunted 
  

   Telephone 
  to 
  the 
  Antenna 
  Current 
  as 
  used 
  in 
  the 
  Reception 
  

   of 
  Wireless 
  Signals. 
  By 
  Prof. 
  G. 
  W. 
  0. 
  Howe, 
  JD.Sc, 
  

   MJ.E.E* 
  

  

  [See 
  paper 
  with 
  the 
  same 
  title 
  by 
  M. 
  van 
  der 
  Pol, 
  vol. 
  xxxiv. 
  p. 
  184.] 
  

  

  f|">HE 
  audibility 
  factor 
  of 
  a 
  radio-telegraph 
  signal 
  is 
  defined 
  

   JL 
  as 
  the 
  ratio 
  of 
  the 
  actual 
  sound-producing 
  current 
  in 
  

   the 
  telephone-receiver 
  to 
  the 
  minimum 
  value 
  to 
  which 
  this 
  

   current 
  could 
  be 
  reduced 
  for 
  the 
  signals 
  to 
  remain 
  just 
  

   readable. 
  It 
  is 
  assumed 
  that 
  the 
  wave-form 
  of 
  the 
  telephone 
  

   current, 
  and 
  therefore 
  also 
  the 
  character 
  of 
  the 
  sound, 
  

   remain 
  the 
  same 
  in 
  the 
  two 
  cases. 
  This 
  ratio 
  is 
  usually 
  

   determined 
  by 
  shunting 
  the 
  telephone-receivers 
  with 
  a 
  non- 
  

   inductive 
  resistance 
  until 
  the 
  signals 
  are 
  only 
  just 
  readable. 
  

   If 
  there 
  is 
  any 
  possibility 
  of 
  the 
  total 
  rectified 
  current 
  being 
  

   affected 
  by 
  the 
  decreased 
  resistance 
  of 
  the 
  detector 
  circuit 
  

   due 
  to 
  the 
  addition 
  of 
  the 
  shunt, 
  a 
  resistance 
  should 
  be 
  

   inserted 
  in 
  series 
  with 
  the 
  shunted 
  receiver 
  to 
  maintain 
  the 
  

   total 
  resistance 
  of 
  the 
  detector 
  circuit 
  approximately 
  constant. 
  

   From 
  the 
  value 
  of 
  the 
  shunt 
  it 
  is 
  then 
  necessary 
  to 
  calculate 
  

   the 
  ratio 
  of 
  the 
  total 
  or 
  joint 
  current 
  to 
  that 
  through 
  the 
  

   receiver. 
  

  

  It 
  is 
  not 
  clear 
  from 
  Mr. 
  van 
  der 
  Pol's 
  paper 
  how 
  he 
  deter- 
  

   mined 
  the 
  resistance 
  of 
  the 
  receiver 
  wbich 
  he 
  gives 
  as 
  

   1240 
  ohms 
  ; 
  but 
  since 
  nothing 
  is 
  stated 
  to 
  the 
  contrary, 
  it 
  

   would 
  appear 
  that 
  he 
  has 
  treated 
  the 
  receiver 
  as 
  a 
  non- 
  

   inductive 
  resistance 
  equal 
  in 
  value 
  to 
  the 
  actual 
  resistance 
  

   of 
  the 
  receiver 
  to 
  continuous 
  current. 
  If 
  so, 
  the 
  results 
  

   obtained 
  will 
  be 
  in 
  error 
  for 
  two 
  reasons 
  : 
  firstly, 
  because 
  

   the 
  effective 
  resistance 
  of 
  a 
  telephone-receiver 
  at 
  the 
  fre- 
  

   quency 
  employed, 
  viz. 
  467, 
  is 
  considerably 
  greater 
  than 
  its 
  

   resistance 
  to 
  continuous 
  current 
  ; 
  and 
  secondly, 
  because 
  an 
  

   alternating 
  current 
  divides 
  between 
  two 
  alternative 
  paths 
  in 
  

   a 
  manner 
  depending 
  on 
  the 
  impedances 
  and 
  not 
  on 
  the 
  

   resistances. 
  

  

  As 
  an 
  example 
  of 
  the 
  magnitude 
  of 
  the 
  error 
  thus 
  intro- 
  

   duced, 
  the 
  following 
  figures 
  may 
  be 
  quoted 
  : 
  a 
  3200-ohm 
  

   receiver 
  had 
  an 
  effective 
  resistance 
  R/- 
  at 
  750 
  cycles 
  per 
  

   second 
  of 
  6200 
  ohms 
  and 
  an 
  impedance 
  Z 
  of 
  9320 
  ohms, 
  

   whilst 
  at 
  a 
  frequency 
  of 
  1000 
  these 
  values 
  were 
  increased 
  to 
  

   7250 
  and 
  11,200 
  ohms 
  respectively. 
  Thus 
  Z 
  750 
  = 
  2'9R 
  and 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  