﻿On 
  the 
  Frequency 
  of 
  Alternating 
  Currents. 
  339 
  

  

  sur 
  la 
  raie 
  verte 
  du 
  thallium, 
  &c.,lorsqu' 
  une 
  minime 
  quantite 
  

   de 
  ces 
  metaux 
  on 
  d'un 
  de 
  leurs 
  sels 
  est 
  placee 
  sur 
  le 
  support 
  

   de 
  charbon. 
  

  

  61 
  Enfin, 
  les 
  armatures 
  coniques 
  de 
  Felectro-aimant 
  etant 
  

   remplacees 
  par 
  les 
  armatures 
  meplates, 
  de 
  maniere 
  que 
  touto 
  

   la 
  longueur 
  de 
  la 
  flamme 
  sodique 
  soit 
  comprise 
  entre 
  ces 
  

   armatures, 
  les 
  raies 
  D 
  x 
  et 
  D 
  2 
  , 
  prealablement 
  renversees 
  et 
  

   elargies, 
  presentent 
  un 
  double 
  renversement 
  (c'est-a-dire 
  l'ap- 
  

   parition 
  d'une 
  raie 
  brillante 
  au 
  milieu 
  de 
  la 
  raie 
  noire 
  elargie), 
  

   lorsque 
  l'electro-aimant 
  est 
  en 
  activite.'' 
  

  

  XXXV 
  II. 
  Determination 
  of 
  the 
  Frequency 
  of 
  Alternating 
  

   Currents. 
  By 
  Carl 
  Kinsley, 
  M.E., 
  A.M.* 
  

  

  IN 
  using 
  alternating 
  currents 
  experimentally 
  it 
  is 
  necessary 
  

   to 
  know 
  their 
  periodicity. 
  The 
  most 
  simple 
  and 
  direct 
  

   methods 
  of 
  measuring 
  self-induction, 
  mutual 
  induction, 
  and 
  

   capacity 
  require 
  this 
  determination. 
  While 
  working 
  on 
  

   another 
  subject 
  I 
  found 
  that 
  this 
  could 
  be 
  done 
  with 
  great 
  

   accuracy 
  by 
  means 
  of 
  a 
  vibrating 
  air- 
  column. 
  If 
  a 
  telephone 
  

   receiver 
  is 
  used 
  on 
  an 
  alternating 
  current 
  circuit, 
  it 
  will 
  

   give 
  a 
  musical 
  note 
  whose 
  fundamental 
  — 
  first 
  harmonic 
  — 
  

   will 
  have 
  the 
  same 
  number 
  of 
  vibrations 
  that 
  the 
  alternating 
  

   current 
  has 
  periods. 
  When 
  the 
  telephone 
  is 
  placed 
  in 
  front 
  

   of 
  a 
  tube 
  of 
  the 
  right 
  length 
  this 
  note 
  will 
  be 
  reinforced. 
  If 
  

   we 
  can 
  compute 
  the 
  velocity 
  of 
  sound 
  in 
  the 
  air 
  in 
  the 
  tube 
  

   and 
  can 
  determine 
  the 
  wave-length 
  of 
  the 
  sound, 
  the 
  number 
  

   of 
  vibrations 
  or 
  periods 
  per 
  second 
  follows 
  immediately 
  ; 
  

  

  where 
  n 
  = 
  number 
  of 
  periods, 
  

  

  X 
  = 
  the 
  wave-length 
  of 
  the 
  note, 
  

   V^the 
  velocity 
  of 
  the 
  sound. 
  

  

  Thus 
  experimentally 
  it 
  is 
  necessary 
  to 
  determine 
  merely 
  

   the 
  length 
  of 
  the 
  tube 
  for 
  the 
  condition 
  of 
  maximum 
  reson- 
  

   ance. 
  

  

  The 
  receiver 
  should 
  be 
  a 
  powerful 
  one 
  and 
  placed 
  at 
  least 
  

   •41 
  diam. 
  from 
  the 
  tube. 
  Tbe 
  tube 
  should 
  be 
  of 
  uniform 
  

   diameter 
  and 
  the 
  piston 
  fit 
  easily 
  ; 
  then 
  all 
  the 
  adjustments 
  

   can 
  be 
  mado 
  by 
  hand. 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  