﻿772 
  Mr. 
  W. 
  A. 
  Jenkins 
  : 
  New 
  Method 
  of 
  Determining 
  the 
  

   Data 
  for 
  Calculation 
  of 
  H. 
  

   Time 
  12.45 
  A.M.-2 
  A.M. 
  January 
  11th, 
  1913. 
  

  

  Time 
  of 
  swing 
  of 
  magnet 
  under 
  influence 
  o£ 
  earth's 
  

   field 
  alone 
  = 
  2-7577 
  seconds. 
  

  

  Time 
  of 
  swing 
  when 
  magnet 
  vibrates 
  under 
  influence 
  

   of 
  earth's 
  field 
  and 
  field 
  of 
  solenoid 
  =2-7632 
  seconds. 
  

  

  E.M.F. 
  of 
  Cadmium 
  cell 
  corrected 
  for 
  temperature 
  

  

  = 
  1-01875 
  volts. 
  

  

  Resistance 
  of 
  Standard 
  similarly 
  corrected 
  

  

  = 
  9-9992 
  ohms. 
  

  

  Current 
  to 
  produce 
  the 
  above 
  field 
  ='014.4-23 
  ampere. 
  

   The 
  time 
  of 
  swing 
  is 
  given 
  by 
  the 
  formula 
  

  

  VMH' 
  

  

  t 
  ~ 
  2r 
  " 
  V 
  MH 
  

   or 
  t 
  2 
  K 
  = 
  constant. 
  

  

  Suppose 
  C 
  = 
  current 
  required 
  to 
  give 
  field 
  exactly 
  equal 
  to 
  H. 
  

   Then 
  t 
  2 
  C 
  = 
  K, 
  where 
  K 
  is 
  constant. 
  When 
  vibrating 
  

   under 
  combined 
  fields 
  we 
  have 
  

  

  

  

  h 
  2 
  (H,- 
  

  

  — 
  H) 
  = 
  constant, 
  

  

  

  

  or 
  ^(d- 
  

  

  -C) 
  = 
  K, 
  

  

  

  

  *i 
  2 
  (Cx- 
  

  

  -C)=* 
  2 
  C; 
  

  

  and 
  

  

  as 
  

  

  we 
  wish 
  to 
  find 
  C 
  we 
  get 
  

  

  

  

  C: 
  

  

  f 
  2 
  

   -tf 
  + 
  t 
  2 
  ^ 
  1 
  ' 
  

  

  Substituting 
  in 
  this 
  equation 
  the 
  values 
  found, 
  viz. 
  

   *=2'7577, 
  ^=.2-763.2, 
  and 
  Ci 
  = 
  '014423, 
  

  

  we 
  get 
  2C 
  =-014452, 
  

  

  C 
  = 
  -007226. 
  

  

  The 
  number 
  of 
  turns 
  per 
  cm. 
  is 
  18-8789 
  

   and 
  cosine 
  a 
  = 
  -99888. 
  

  

  Substituting 
  these 
  values 
  in 
  the 
  formula 
  

   H 
  = 
  *47rftCcos 
  ol, 
  

   we 
  find 
  that 
  H 
  = 
  -17186. 
  

  

  Three 
  observations 
  of 
  the 
  period 
  of 
  swing 
  under 
  different 
  

  

  