﻿48 
  Rowland 
  and 
  Penniman 
  — 
  Electrical 
  Measurements. 
  

  

  the 
  impedances 
  of 
  the 
  two 
  circuits, 
  as 
  other 
  things 
  being 
  equal 
  

   the 
  smaller 
  current 
  should 
  go 
  through 
  the 
  hanging 
  coil. 
  

  

  Experiments 
  — 
  The 
  coils 
  used 
  in 
  the 
  exjDeriments 
  were 
  coils 
  

   Pj, 
  Pj, 
  C, 
  Bj, 
  Bj, 
  and 
  A, 
  which 
  coils 
  are 
  described 
  on 
  page 
  36. 
  

   From 
  the 
  dimensions 
  of 
  Pj 
  and 
  its 
  self-inductance 
  as 
  found 
  by 
  

   Method 
  25, 
  B^ 
  was 
  designed 
  to 
  have 
  a 
  self-inductance 
  of 
  one 
  

   henry. 
  This 
  will 
  be 
  shown 
  to 
  be 
  nearly 
  the 
  case. 
  For 
  ease 
  

   of 
  comparison 
  B^ 
  has 
  been 
  taken 
  in 
  the 
  calculations 
  of 
  the 
  

   results 
  as 
  being 
  equal 
  to 
  one 
  henry, 
  and 
  the 
  other 
  coils 
  were 
  

   compared 
  with 
  this 
  coil 
  as 
  a 
  standard. 
  

  

  In 
  these 
  experiments 
  the 
  connections 
  were 
  made 
  as 
  in 
  the 
  

   figure 
  47, 
  the 
  coil 
  B^ 
  that 
  was 
  taken 
  as 
  the 
  standard 
  being 
  

   placed 
  in 
  circuit 
  with 
  the 
  fixed 
  coils 
  of 
  the 
  electrodj^na- 
  

   mometer 
  as 
  Jj" 
  and 
  the 
  resistance 
  of 
  this 
  circuit 
  was 
  unaltered 
  

   during 
  the 
  experiments 
  in 
  any 
  particular 
  series. 
  The 
  coils 
  

   whose 
  self 
  -inductances 
  were 
  to 
  be 
  determined 
  were 
  placed 
  in 
  

   the 
  hanging 
  coil 
  circuit 
  and 
  the 
  resistance 
  P 
  was 
  changed 
  until 
  

   there 
  was 
  no 
  deflection. 
  The 
  resistance 
  of 
  the 
  two 
  circuits, 
  

   ^" 
  and 
  P+r 
  were 
  then 
  measured 
  by 
  a 
  Wheatstone 
  bridge. 
  

  

  The 
  resistance 
  r 
  was 
  in 
  all 
  cases 
  small 
  in 
  order 
  that 
  C^e^'^i 
  

   should 
  be 
  large, 
  and 
  therefore 
  by 
  induction 
  C^£i{^i 
  + 
  <P) 
  the 
  cur- 
  

   rent 
  through 
  the 
  fixed 
  coils 
  was 
  made 
  large 
  and 
  the 
  instrument 
  

   sensitive. 
  The 
  method 
  being 
  very 
  accurate, 
  as 
  will 
  be 
  seen 
  

   later, 
  great 
  care 
  had 
  to 
  be 
  used 
  to 
  eliminate 
  all 
  sources 
  of 
  error, 
  

   as 
  for 
  example, 
  electrostatic 
  action. 
  In 
  the 
  first 
  trial 
  of 
  the 
  

   method 
  small 
  differences 
  were 
  noticed 
  in 
  the 
  ratio 
  of 
  two 
  self- 
  

   inductances, 
  depending 
  both 
  on 
  the 
  resistances 
  used, 
  and 
  also 
  

   on 
  the 
  connections 
  of 
  the 
  coils, 
  whether 
  the 
  leads 
  were 
  double, 
  

   single, 
  long 
  or 
  short. 
  The 
  same 
  variation 
  was 
  noticed 
  when 
  

   several 
  coils 
  were 
  joined 
  in 
  series 
  and 
  compared 
  with 
  another 
  

   coil, 
  and 
  when 
  these 
  coils 
  were 
  compared 
  separately 
  and 
  their 
  

   sum 
  taken. 
  

  

  This 
  irregularity 
  led 
  to 
  an 
  investigation 
  of 
  the 
  effects 
  of 
  

   various 
  resistances 
  and 
  connections 
  in 
  one 
  of 
  the 
  circuits, 
  the 
  

   other 
  circuit 
  being 
  unaltered. 
  A 
  little 
  farther 
  on, 
  the 
  varia- 
  

   tion 
  in 
  the 
  deduced 
  value 
  of 
  the 
  self-inductance 
  of 
  one 
  of 
  the 
  

   coils, 
  when 
  different 
  resistances 
  and 
  leads 
  were 
  used, 
  will 
  be 
  

   given, 
  which 
  variation 
  was 
  caused 
  by 
  the 
  electrostatic 
  action 
  

   of 
  the 
  connections, 
  etc. 
  (Page 
  37.) 
  

  

  The 
  necessity 
  of 
  eliminating 
  electrostatic 
  action 
  made 
  obliga- 
  

   tory 
  the 
  use 
  of 
  open 
  resistances 
  which 
  had 
  small 
  self-induc- 
  

   tances. 
  These 
  resistances 
  were 
  of 
  three 
  kinds 
  — 
  resistances 
  in 
  

   the 
  form 
  of 
  spirals, 
  resistances 
  wound 
  on 
  thin 
  strips 
  of 
  mica- 
  

   nite 
  or 
  paper, 
  and 
  those 
  wound 
  on 
  open 
  frames 
  ; 
  see 
  page 
  37. 
  

  

  The 
  self 
  inductance 
  of 
  the 
  first 
  and 
  second 
  classes 
  of 
  resist- 
  

   ances 
  was 
  very 
  small, 
  as 
  in 
  one 
  case 
  there 
  were 
  only 
  a 
  few 
  

   turns, 
  and 
  in 
  the 
  other 
  the 
  cross-section 
  was 
  very 
  small. 
  

  

  