﻿Variable 
  Frequency 
  Alternating 
  Currents. 
  481 
  

  

  apparatus 
  should 
  be 
  about 
  1 
  watt, 
  and 
  it 
  is 
  necessary 
  that 
  it 
  

   should 
  be 
  capable 
  of 
  working 
  uniformly 
  over 
  a 
  fairly 
  con- 
  

   siderable 
  period 
  of 
  time. 
  

  

  Simplicity 
  and 
  portability 
  are 
  also 
  highly 
  desirable. 
  

  

  Alternators. 
  — 
  A 
  sine 
  wave 
  alternator 
  coupled 
  to 
  a 
  motor, 
  

   the 
  combination 
  being 
  capable 
  of 
  running 
  at 
  a 
  constant 
  speed 
  

   for 
  a 
  considerable 
  period, 
  is 
  a 
  very 
  useful 
  form 
  of 
  apparatus 
  

   for 
  investigations 
  in 
  connexion 
  with 
  telephony. 
  A 
  suitable 
  

   machine 
  is, 
  however, 
  difficult 
  to 
  obtain. 
  

  

  The 
  Western 
  Electric 
  Company 
  build 
  a 
  machine 
  with 
  an 
  

   output 
  of 
  about 
  30 
  watts 
  at 
  frequencies 
  varying 
  from 
  800^ 
  

   to 
  1800 
  — 
  , 
  and 
  the 
  wave 
  form 
  is 
  stated 
  to 
  resemble 
  a 
  sine 
  

   curve 
  closely 
  at 
  all 
  loads. 
  

  

  Messrs. 
  Siemens 
  and 
  Halske 
  also 
  make 
  a 
  machine 
  with 
  an 
  

   output 
  of 
  3 
  or 
  4 
  watts 
  at 
  about 
  the 
  same 
  frequencies. 
  Both 
  

   these 
  machines 
  are 
  of 
  the 
  inductor 
  type, 
  the 
  purity 
  of 
  the 
  

   wave 
  form, 
  being 
  secured 
  by 
  the 
  shape 
  of 
  the 
  teeth 
  and 
  pole 
  

   faces. 
  

  

  Mr. 
  DuddelPs 
  work 
  with 
  high 
  frequency 
  alternators 
  is 
  

   well 
  known. 
  He 
  has 
  also 
  made 
  several 
  medium 
  frequency 
  

   machines, 
  one 
  of 
  which, 
  with 
  a 
  wound 
  rotor, 
  gives 
  an 
  output 
  

   of 
  about 
  20 
  watts, 
  but 
  only 
  runs 
  up 
  to 
  about 
  500-^-. 
  

  

  In 
  the 
  Investigation 
  Department 
  of 
  the 
  National 
  Telephone 
  

   Company 
  we 
  have 
  built 
  a 
  small 
  alternator 
  of 
  the 
  inductor 
  

   type 
  on 
  the 
  lines 
  of 
  the 
  Siemens 
  machine. 
  This 
  is 
  illus- 
  

   trated 
  in 
  PL 
  XYI. 
  fig. 
  1. 
  

  

  With 
  a 
  30 
  volt 
  battery 
  connected 
  to 
  the 
  field 
  windings, 
  the 
  

   output 
  is 
  only 
  about 
  *3 
  watt, 
  but 
  the 
  wave 
  form 
  is 
  fairly 
  good, 
  

   i. 
  e. 
  it 
  approximates 
  to 
  a 
  sine 
  wave. 
  By 
  decreasing 
  the 
  air 
  

   gap, 
  the 
  output 
  can 
  be 
  considerably 
  increased 
  at 
  the 
  expense 
  

   of 
  the 
  purity 
  of 
  the 
  wave 
  form. 
  

  

  With 
  the 
  best 
  air 
  gap 
  (about 
  *015 
  mm.) 
  the 
  P.D., 
  which 
  

   is 
  independent 
  of 
  the 
  frequency, 
  is 
  10*5 
  volts. 
  

  

  Fig. 
  2 
  (PL 
  XVI.) 
  shows 
  the 
  wave 
  form 
  of 
  this 
  machine 
  

   when 
  the 
  frequency 
  is 
  800~. 
  Measurements 
  of 
  the 
  capacity 
  

   of 
  small 
  mica 
  condensers 
  made 
  with 
  the 
  current 
  from 
  this 
  alter- 
  

   nator 
  on 
  the 
  assumption 
  that 
  it 
  is 
  a 
  pure 
  sine 
  wave, 
  are 
  accurate 
  

   to 
  within 
  about 
  3 
  per 
  cent., 
  and 
  this 
  forms 
  a 
  very 
  good 
  test. 
  

  

  For 
  more 
  accurate 
  measurements 
  this 
  machine 
  has 
  been 
  

   used 
  with 
  a 
  wave 
  filter, 
  consisting 
  of 
  series 
  inductances 
  of 
  low 
  

   effective 
  resistance 
  and 
  parallel 
  capacities, 
  similar 
  to 
  that 
  

   described 
  by 
  G. 
  A. 
  Campbell 
  (Phil. 
  Mag. 
  March 
  1903) 
  . 
  The 
  

   w 
  r 
  ave 
  form 
  when 
  at 
  a 
  frequency 
  of 
  800^- 
  this 
  filter 
  is 
  used 
  is 
  

   shown 
  in 
  fig. 
  3 
  (PL 
  XVI.). 
  

  

  Humming 
  Telephone. 
  — 
  The 
  interaction 
  between 
  a 
  receiver 
  

   and 
  a 
  transmitter 
  setting 
  up 
  an 
  alternating 
  current 
  whose 
  

  

  Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  1G. 
  No. 
  93. 
  Sept. 
  1908. 
  2 
  K 
  

  

  