﻿804 
  Dr. 
  Russell 
  and 
  Mr. 
  Altj 
  : 
  Electromagnetic 
  Metliod 
  

   We 
  thus 
  obtain 
  

  

  /w 
  

  

  where, 
  by 
  (2), 
  

   and 
  

  

  = 
  an 
  + 
  

  

  Ai 
  

  

  + 
  

  

  x 
  — 
  hi 
  X 
  — 
  b 
  

  

  + 
  

  

  X 
  — 
  hn 
  

  

  ■ 
  (3) 
  

  

  A1 
  + 
  A2 
  + 
  

  

  f{K 
  

  

  + 
  A„=0, 
  . 
  

  

  {bi-h,){b,-h) 
  

  

  and 
  

  

  

  (^) 
  

  

  (5) 
  

  

  Let 
  us 
  now 
  consider 
  the 
  magnetic 
  field 
  round 
  a 
  long 
  

   vertical 
  wire 
  carrying 
  a 
  current 
  <>£ 
  amperes, 
  and 
  let 
  us 
  

   suppose 
  that 
  the 
  earth's 
  horizontal 
  field 
  in 
  the 
  neighbourhood 
  

   is 
  uniform, 
  and 
  that 
  its 
  horizontal 
  intensity 
  in 
  c.G.s. 
  units 
  

   is 
  H. 
  The 
  magnetic 
  force 
  at 
  any 
  point 
  P 
  at 
  a 
  perpendicular 
  

   distance 
  of 
  r 
  centimetres 
  from 
  the 
  axis 
  of 
  the 
  wire 
  will 
  be 
  

   the 
  resultant 
  of 
  a 
  force 
  Q/^r 
  acting 
  at 
  right 
  angles 
  to 
  the 
  

   plane 
  containing 
  r 
  and 
  the 
  axis 
  of 
  the 
  wire 
  and 
  a 
  force 
  H 
  

   directed 
  to 
  the 
  magnetic 
  pole. 
  There 
  is 
  always 
  a 
  neutral 
  

   point 
  "^ 
  on 
  the 
  line 
  through 
  the 
  axis 
  of 
  the 
  wire 
  perpen- 
  

   dicular 
  to 
  the 
  magnetic 
  meridian. 
  If 
  x 
  be 
  the 
  distance 
  of 
  

   this 
  point 
  from 
  the 
  axis 
  

  

  ^=C/(5H) 
  (6) 
  

  

  This 
  formula 
  is 
  utilized 
  in 
  a 
  well-known 
  rough 
  laboratory 
  

   method 
  of 
  measuring 
  H 
  when 
  C 
  is 
  known 
  or 
  vice 
  versa. 
  

  

  Let 
  us 
  now 
  suppose 
  that 
  we 
  have 
  n 
  vertical 
  wires 
  arranged 
  

   in 
  a 
  plane 
  perpendicular 
  to 
  the 
  magnetic 
  meridian, 
  and 
  let 
  

  

  Figr. 
  1. 
  

  

  

  Y 
  

  

  x^^ 
  i 
  

  

  P 
  

  

  

  ' 
  

  

  

  / 
  

  

  [ 
  

  

  / 
  

  

  

  

  

  H 
  

  

  

  / 
  

  

  

  

  

  

  

  

  

  B, 
  h 
  

  

  ^ 
  

  

  B. 
  

  

  4 
  

  

  X 
  

  

  them 
  cut 
  the 
  plane 
  of 
  the 
  paper 
  perpendicularly 
  at 
  {Bi, 
  

   B2, 
  :.. 
  B„ 
  (fig. 
  1) 
  which 
  are 
  at 
  distances 
  hi, 
  Z>2, 
  ... 
  hn 
  from 
  0. 
  

  

  * 
  A. 
  Russell, 
  ' 
  The 
  Electrician,' 
  vol. 
  xxxi. 
  p. 
  282 
  (1893). 
  

  

  