﻿Measurement 
  by 
  Alternating 
  Currents, 
  71 
  

  

  currents 
  generate 
  electromagnetic 
  waves 
  which 
  are 
  so 
  strong 
  

   that 
  currents 
  exist 
  in 
  every 
  closed 
  circuit 
  with 
  any 
  opening 
  

   between 
  conductors 
  in 
  the 
  vicinity. 
  

  

  We 
  eliminate 
  this 
  source 
  of 
  error 
  by 
  twisting 
  wires 
  together 
  

   and 
  other 
  expedients. 
  But 
  in 
  avoiding 
  one 
  error, 
  we 
  plunge 
  

   into 
  another. 
  For, 
  by 
  twisting 
  wires 
  we 
  introduce 
  electro- 
  

   static 
  capacity 
  between 
  them, 
  which 
  may 
  vitiate 
  our 
  results. 
  

   Thus, 
  in 
  methods 
  23 
  or 
  24 
  for 
  comparing 
  mutual 
  inductances, 
  

   if 
  there 
  is 
  electrostatic 
  capacity 
  between 
  the 
  wires, 
  a 
  current 
  

   will 
  flow 
  through 
  the 
  electrodynamometer 
  in 
  the 
  testing 
  

   circuit 
  and 
  destroy 
  the 
  balance. 
  

  

  Various 
  expedients 
  suggest 
  themselves 
  to 
  eliminate 
  this 
  

   trouble, 
  as, 
  for 
  instance, 
  the 
  variation 
  of 
  the 
  resistance 
  A 
  in 
  

   the 
  above 
  ; 
  but 
  I 
  shall 
  reserve 
  them 
  for 
  a 
  future 
  paper. 
  I 
  

   may 
  say, 
  however, 
  that 
  it 
  is 
  sometimes 
  possible, 
  as 
  in 
  method 
  

   12 
  for 
  instance, 
  to 
  choose 
  a 
  method 
  in 
  which 
  the 
  error 
  does 
  

   not 
  exist. 
  

  

  However, 
  with 
  the 
  very 
  best 
  method, 
  much 
  rests 
  with 
  the 
  

   experimenter, 
  as 
  errors 
  from 
  electromagnetic 
  and 
  electrostatic 
  

   induction 
  are 
  added 
  to 
  errors 
  from 
  defective 
  insulation 
  when 
  

   we 
  use 
  alternating 
  currents. 
  

  

  These 
  errors 
  are 
  generally 
  less 
  than 
  one 
  per 
  cent., 
  however, 
  

   and 
  intelligent 
  and 
  careful 
  work 
  reduces 
  them 
  to 
  less 
  than 
  this. 
  

  

  The 
  following 
  methods 
  generally 
  refer 
  by 
  number 
  to 
  Plate 
  

   XIX., 
  on 
  which 
  the 
  resistances 
  &c. 
  are 
  generally 
  marked. 
  

   One 
  large 
  circle 
  with 
  a 
  small 
  one 
  inside 
  represents 
  an 
  electro- 
  

   dynamometer. 
  Of 
  course 
  the 
  circuit 
  of 
  the 
  small 
  coil 
  can 
  be 
  

   interchanged 
  with 
  the 
  large 
  one. 
  Generally 
  we 
  make 
  the 
  

   smaller 
  current 
  go 
  through 
  the 
  hanging 
  coil. 
  

  

  By 
  the 
  methods 
  1 
  to 
  14, 
  we 
  adjust 
  the 
  electrodynamometer 
  

   to 
  zero 
  by 
  making 
  the 
  phase-difference 
  in 
  the 
  two 
  coils 
  90°. 
  

   For 
  greatest 
  sensitiveness, 
  the 
  currents 
  through 
  the 
  two 
  coils 
  

   must 
  be 
  the 
  greatest 
  possible, 
  heating 
  being 
  the 
  limit. 
  This 
  

   current 
  should 
  be 
  first 
  calculated 
  from 
  the 
  impedance 
  of 
  the 
  

   circuit, 
  as 
  there 
  is 
  danger 
  of 
  making 
  it 
  too 
  great. 
  

  

  In 
  the 
  second 
  series 
  of 
  methods, 
  15-26, 
  the 
  branch 
  circuit 
  

   in 
  which 
  the 
  current 
  is 
  to 
  be 
  is 
  indicated 
  by 
  0. 
  

  

  Resistances 
  in 
  the 
  separate 
  circuits 
  are 
  represented 
  by 
  R 
  R/ 
  

   R, 
  &c, 
  and 
  r 
  r' 
  r, 
  &c. 
  Corresponding 
  self-inductances 
  and 
  

   capacities 
  in 
  the 
  same 
  circuits 
  are 
  L 
  1/ 
  L, 
  &c, 
  and 
  I 
  V 
  I, 
  &c, 
  

   or 
  C' 
  C, 
  &c, 
  and 
  c 
  d 
  e, 
  &c. 
  b 
  = 
  27rn, 
  where 
  n 
  is 
  the 
  

   number 
  of 
  complete 
  current 
  waves 
  per 
  second. 
  

  

  The 
  currents 
  must 
  be 
  as 
  heavy 
  as 
  possible, 
  j 
  1 
  ^ 
  ampere 
  or 
  

   more, 
  and 
  it 
  is 
  well 
  to 
  make 
  those 
  that 
  require 
  a 
  current 
  of 
  

   more 
  than 
  t 
  Jq 
  ampere 
  of 
  larger 
  wire 
  freely 
  suspended 
  in 
  oil. 
  

   A 
  larger 
  current 
  can, 
  however, 
  be 
  passed 
  through 
  an 
  ordinary 
  

  

  