﻿66 
  Prof. 
  H. 
  A. 
  Rowland 
  on 
  Electrical 
  

  

  V. 
  Electrical 
  Measurement 
  by 
  Alternating 
  Currents, 
  By 
  

   Henry 
  A. 
  Rowland, 
  Johns 
  Hopkins 
  University, 
  Balti- 
  

   more, 
  Md* 
  

  

  [Plate 
  XIX.] 
  

  

  THE 
  electrical 
  quantities 
  pertaining 
  to 
  an 
  electric 
  current 
  

   which 
  it 
  is 
  usually 
  necessary 
  to 
  measure, 
  in 
  addition 
  to 
  

   current, 
  electromotive 
  force, 
  wafcts^ 
  &c, 
  are 
  resistances, 
  self 
  

   and 
  mutual 
  inductances, 
  and 
  capacities. 
  I 
  propose 
  to 
  treat 
  

   of 
  the 
  measurement 
  of 
  alternating 
  currents, 
  electromotive 
  

   force, 
  and 
  watts 
  in 
  a 
  separate 
  paper. 
  Resistances 
  are 
  ordi- 
  

   narily 
  best 
  dealt 
  with 
  by 
  continuous 
  currents, 
  except 
  liquid 
  

   resistances. 
  I 
  propose 
  to 
  treat 
  in 
  this 
  paper, 
  however, 
  mainly 
  

   of 
  inductances, 
  self 
  and 
  mutual, 
  and 
  of 
  capacities 
  together 
  

   with 
  their 
  ratios 
  and 
  values 
  in 
  absolute 
  measure 
  as 
  obtained 
  

   by 
  alternating 
  currents. 
  I 
  also 
  give 
  a 
  few 
  methods 
  of 
  re- 
  

   sistance 
  measurement 
  more 
  accurate 
  than 
  usually 
  given 
  by 
  

   means 
  of 
  telephones 
  or 
  electrodynamometer 
  as 
  commonly 
  used 
  

   and 
  specially 
  suitable 
  for 
  resistances 
  of 
  electrolytic 
  liquids. 
  

  

  I 
  have 
  introduced 
  many 
  new 
  and 
  some 
  old 
  methods 
  de- 
  

   pending 
  upon 
  making 
  the 
  whole 
  current 
  through 
  a 
  given 
  

   branch 
  circuit 
  equal 
  to 
  zero. 
  These 
  always 
  require 
  two 
  'ad- 
  

   justments 
  and 
  they 
  must 
  often 
  be 
  made 
  simultaneously. 
  

   However, 
  some 
  of 
  them 
  admit 
  of 
  the 
  adjustments 
  being 
  made 
  

   independently 
  of 
  each 
  other, 
  and 
  these, 
  of 
  course, 
  are 
  the 
  

   most 
  convenient. 
  But 
  none 
  of 
  these 
  zero 
  methods 
  admit 
  

   of 
  any 
  great 
  accuracy 
  unless 
  very 
  heavy 
  currents 
  are 
  passed 
  

   through 
  the 
  resistances. 
  The 
  reason 
  of 
  this 
  is 
  that 
  an 
  electro- 
  

   dynamometer 
  cannot 
  be 
  made 
  nearly 
  as 
  sensitive 
  for 
  small 
  

   currents 
  as 
  a 
  magnetic 
  galvanometer. 
  The 
  deflexion 
  of 
  an 
  

   electrodynamometer 
  is 
  as 
  the 
  square 
  of 
  the 
  current. 
  To 
  make 
  

   it 
  doubly 
  sensitive 
  requires 
  double 
  the 
  number 
  of 
  turns 
  in 
  

   both 
  the 
  coils. 
  Hence 
  we 
  quickly 
  reach 
  a 
  limit 
  of 
  sensitive^ 
  

   ness. 
  It 
  is 
  easy 
  to 
  measure 
  an 
  alternating 
  current 
  of 
  '0001 
  

   ampere 
  and 
  difficult 
  for 
  *00001 
  ampere. 
  A 
  telephone 
  is 
  more 
  

   sensitive 
  and 
  an 
  instrument 
  made 
  by 
  suspending 
  a 
  piece 
  of 
  

   soft 
  iron 
  at 
  an 
  angle 
  of 
  45°, 
  as 
  invented 
  by 
  Lord 
  Rayleigh, 
  

   is 
  also 
  probably 
  more 
  sensitive. 
  

  

  For 
  this 
  reason 
  I 
  have 
  introduced 
  here 
  many 
  new 
  methods 
  

   depending 
  upon 
  adjusting 
  two 
  currents 
  to 
  a 
  phase-difference 
  

   of 
  90°, 
  which 
  1 
  believe 
  to 
  be 
  a 
  new 
  principle. 
  This 
  I 
  do 
  by 
  

   passing 
  one 
  current 
  through 
  the 
  fixed 
  and 
  the 
  other 
  through 
  

   the 
  suspended 
  coil 
  of 
  an 
  electrodynamometer. 
  By 
  this 
  means 
  

   a 
  heavy 
  current 
  can 
  be 
  passed 
  through 
  the 
  fixed 
  coils 
  and 
  a 
  

  

  * 
  Communicated 
  by 
  the 
  Author. 
  

  

  