﻿762 
  Mr. 
  W* 
  A. 
  Jenkins 
  : 
  New 
  Method 
  of 
  Determining 
  the 
  

  

  Accuracy 
  of 
  the 
  Experiment. 
  

  

  The 
  different 
  parts 
  of 
  the 
  apparatus 
  were 
  designed 
  with 
  

   u 
  view 
  of 
  attaining 
  an 
  accuracy 
  in 
  the 
  value 
  of 
  H 
  obtained, 
  

   of 
  1 
  part 
  in 
  10,000. 
  Errors 
  may 
  be 
  introduced 
  by 
  the 
  

   following 
  factors: 
  — 
  

  

  (1) 
  The 
  magnetic 
  axis 
  of 
  the 
  solenoid 
  not 
  being 
  perfectly 
  

  

  horizontal. 
  

  

  (2) 
  The 
  magnetic 
  axis 
  of 
  the 
  tube 
  not 
  being 
  perfectly 
  

  

  parallel 
  to 
  the 
  direction 
  of 
  H. 
  

  

  (3) 
  Irregularity 
  in 
  the 
  winding 
  causing 
  the 
  formula 
  

  

  H 
  = 
  '^ttiiC 
  cos 
  a 
  to 
  be 
  at 
  fault. 
  

  

  (4) 
  A 
  longitudinal 
  displacement 
  of 
  the 
  magnet 
  from 
  the 
  

  

  centre 
  of 
  the 
  tube. 
  

  

  (5) 
  A 
  residual 
  torsion 
  in 
  the 
  fibre. 
  

  

  (6) 
  Inaccurate 
  determination 
  of 
  the 
  period 
  of 
  oscillation. 
  

   '(7) 
  Uncertainty 
  in 
  the 
  determination 
  of 
  the 
  current, 
  

  

  v(8) 
  An 
  error 
  in 
  the 
  determination 
  of 
  the 
  number 
  of 
  turns 
  

  

  per 
  cm. 
  

   (9) 
  Heating 
  of 
  the 
  coil 
  by 
  the 
  passage 
  of 
  the 
  current. 
  

  

  How 
  each 
  of 
  these 
  factors 
  will 
  affect 
  the 
  current 
  will 
  now 
  

   be 
  discussed. 
  

  

  (1) 
  The 
  magnetic 
  axis 
  of 
  the 
  solenoid- 
  not 
  being 
  

   horizontal. 
  

  

  If 
  the 
  magnetic 
  axis 
  make 
  an 
  angle 
  a 
  with 
  the 
  horizontal, 
  

   then 
  the 
  force 
  acting 
  on 
  the 
  magnet 
  will 
  be 
  % 
  cos 
  a 
  instead 
  

   of 
  %, 
  and 
  an 
  error 
  would 
  be 
  introduced 
  in 
  regarding 
  it 
  as 
  %. 
  

   The 
  ebonite-covered 
  copper 
  tube 
  was 
  turned 
  down 
  to 
  a 
  

   perfect 
  cylinder 
  in 
  a 
  reliable 
  lathe, 
  and 
  as 
  the 
  cutting 
  of 
  the 
  

   screw-thread 
  was 
  also 
  done 
  by 
  a 
  good 
  machine, 
  there 
  is 
  no 
  

   danger 
  of 
  the 
  helix 
  not 
  being 
  a 
  true 
  cylindrical 
  one. 
  Such 
  

   being 
  the 
  case, 
  then 
  the 
  magnetic 
  axis 
  of 
  the 
  tube 
  will 
  be 
  

   accurately 
  parallel 
  to 
  the 
  outside 
  surface 
  of 
  the 
  helix, 
  and 
  

   if 
  the 
  latter 
  be 
  horizontal, 
  then 
  the 
  former 
  will 
  be 
  also. 
  

  

  The 
  setting 
  of 
  the 
  solenoid 
  in 
  a 
  horizontal 
  plane 
  is 
  not 
  

   difficult, 
  and 
  can 
  easily 
  be 
  performed 
  so 
  as 
  to 
  make 
  a. 
  

   neo-liaible. 
  

  

  o 
  

  

  (2) 
  Error 
  due 
  to 
  the 
  magnetic 
  axis 
  not 
  being 
  parallel 
  

   to 
  the 
  direction 
  of 
  H. 
  

  

  The 
  condition 
  to 
  be 
  fulfilled 
  in 
  order 
  that 
  this 
  factor 
  shall 
  

   be 
  of 
  non-effect 
  is 
  that 
  cos 
  a 
  = 
  l 
  shall 
  be 
  correct 
  to 
  at 
  least 
  

   1 
  part 
  in 
  100,000 
  ; 
  where 
  a. 
  is 
  the 
  angle 
  between 
  the 
  two 
  

   directions. 
  The 
  following 
  experiment 
  was 
  carried 
  out 
  in 
  

  

  