﻿72 
  Prof. 
  H. 
  A. 
  Rowland 
  on 
  Electrical 
  

  

  resistance-box 
  for 
  a 
  second 
  or 
  two 
  without 
  danger. 
  A 
  few 
  

   fixed 
  coarse 
  resistances 
  of 
  large 
  wire 
  in 
  air 
  or 
  oil 
  with 
  ordinary 
  

   resistance-boxes 
  for 
  fine 
  adjustment, 
  are 
  generally 
  all 
  that 
  

   are 
  required. 
  Special 
  boxes 
  avoiding 
  electrostatic 
  induction 
  

   are, 
  however, 
  the 
  best, 
  but 
  are 
  not 
  now 
  generally 
  obtainable. 
  

   In 
  some 
  methods, 
  such 
  as 
  8, 
  9, 
  10, 
  &c., 
  we 
  can 
  eliminate 
  

   undesirable 
  terms 
  containing 
  the 
  current 
  period 
  by 
  using 
  a 
  

   key 
  which 
  suddenly 
  changes 
  the 
  connexions 
  before 
  the 
  period 
  

   has 
  time 
  to 
  change 
  much. 
  

  

  In 
  using 
  twisted-wire 
  mutual 
  inductances, 
  methods 
  7 
  and 
  12 
  

   are 
  about 
  or 
  entirely 
  free 
  from 
  error 
  due 
  to 
  electrostatic 
  action 
  

   between 
  the 
  wires. 
  In 
  all 
  the 
  methods 
  this 
  error 
  is 
  less 
  when 
  

   the 
  resistance 
  of 
  the 
  coils 
  is 
  least, 
  and 
  in 
  23 
  and 
  24 
  when 
  A 
  

   is 
  least. 
  In 
  method 
  8 
  the 
  error 
  is 
  very 
  small 
  when 
  the 
  coil- 
  

   resistances 
  and 
  R 
  are 
  small 
  and 
  r 
  great. 
  In 
  this 
  method 
  with 
  

   1 
  henry 
  and 
  1 
  microfarad, 
  the 
  error 
  need 
  not 
  exceed 
  1 
  in 
  

   1000. 
  Probably 
  the 
  same 
  remarks 
  apply 
  to 
  9, 
  10, 
  11 
  also. 
  

   By 
  suitable 
  adjustment 
  of 
  resistances 
  in 
  the 
  other 
  method, 
  

   the 
  error 
  may 
  be 
  reduced 
  to 
  a 
  minimum. 
  It 
  can, 
  of 
  course, 
  

   be 
  calculated 
  and 
  corrected 
  for. 
  

  

  An 
  electrodynamometer 
  can 
  be 
  made 
  to 
  detect 
  '0001 
  ampere 
  

   without 
  making 
  the 
  self-inductance 
  of 
  the 
  suspended 
  coil 
  more 
  

   than 
  "0007 
  henry 
  or 
  that 
  of 
  the 
  stationary 
  coils 
  more 
  than 
  '0006 
  

   henry, 
  the 
  latter 
  coil 
  readily 
  sustaining 
  a 
  current 
  of 
  j- 
  1 
  ^ 
  ampere 
  

   without 
  much 
  heating. 
  

  

  An 
  error 
  may 
  creep 
  in 
  by 
  methods 
  1-14 
  if 
  the 
  current 
  

   through 
  the 
  suspension 
  is 
  too 
  great, 
  thus 
  heating 
  it 
  and 
  

   possibly 
  twisting 
  it. 
  This 
  should 
  be 
  tested 
  by 
  short-circuiting 
  

   the 
  suspended 
  coil 
  or 
  varying 
  the 
  current. 
  This 
  error 
  can 
  be 
  

   eliminated 
  by 
  always 
  observing 
  by 
  reversing 
  the 
  current 
  in 
  

   one 
  coil. 
  

  

  \ 
  Inductances 
  containing 
  iron 
  introduce 
  harmonics 
  and 
  vary 
  

   with 
  current 
  strength. 
  Thus 
  they 
  have 
  no 
  fixed 
  value. 
  

  

  Closed 
  circuits 
  or 
  masses 
  of 
  metal 
  near 
  a 
  self-inductance 
  

   diminish 
  it, 
  and 
  increase 
  the 
  apparent 
  resistance, 
  which 
  effects 
  

   vary 
  with 
  the 
  period. 
  Short 
  circuits 
  in 
  coils 
  are 
  thus 
  detected. 
  

  

  Electrolytic 
  cells 
  act 
  as 
  capacities 
  which, 
  as 
  well 
  as 
  the 
  

   apparent 
  resistance, 
  vary 
  with 
  the 
  current 
  period. 
  They 
  also 
  

   introduce 
  harmonics. 
  The 
  same 
  may 
  be 
  said 
  of 
  an 
  electric 
  arc. 
  

  

  An 
  incandescent 
  lamp 
  or 
  hot 
  wire 
  introduces 
  harmonics 
  

   into 
  the 
  circuit. 
  

  

  Hysteresis 
  in 
  an 
  iron 
  inductance 
  acts 
  as 
  an 
  apparent 
  re- 
  

   sistance 
  in 
  the 
  wire 
  almost 
  independent 
  of 
  the 
  current 
  period 
  

   and 
  does 
  not, 
  of 
  itself, 
  introduce 
  harmonics. 
  The 
  harmonics 
  

   are 
  due 
  to 
  the 
  variation 
  of 
  the 
  magnetic 
  permeability 
  with 
  

   the 
  amount 
  of 
  magnetization. 
  

  

  