﻿768 
  Mr. 
  W. 
  A. 
  Jenkins 
  : 
  New 
  Method 
  of 
  Determining 
  the 
  

  

  magnetized 
  clock-spring 
  can 
  be 
  placed 
  in 
  the 
  holder. 
  This 
  

   both 
  lengthens 
  the 
  time 
  during 
  which 
  it 
  will 
  oscillate 
  and 
  

   diminishes 
  the 
  angle 
  " 
  a" 
  discussed 
  in 
  the 
  last 
  section. 
  

  

  The 
  eye-and-ear 
  method 
  was 
  of 
  course 
  adopted 
  in 
  the 
  

   actual 
  experiment. 
  

  

  In 
  making 
  another 
  apparatus 
  it 
  would 
  be 
  better 
  to 
  use 
  

   a 
  wider 
  tube 
  and 
  so 
  permit 
  the 
  lengthening 
  of 
  the 
  silk 
  

   fibre. 
  

  

  The 
  increased 
  accuracy 
  with 
  which 
  the 
  oscillation 
  period 
  

   could 
  be 
  found 
  would 
  more 
  than 
  compensate 
  for 
  the 
  increased 
  

   importance 
  of 
  the 
  factor 
  cos 
  a 
  in 
  the 
  formula 
  

  

  H 
  = 
  47r^C 
  cos 
  a. 
  

  

  Lengthening 
  the 
  tube 
  would 
  be 
  no 
  disadvantage 
  if 
  the 
  

   apparatus 
  were 
  to 
  be 
  fixed 
  permanently 
  in 
  one 
  place, 
  and 
  this 
  

   would 
  permit 
  of 
  an 
  increased 
  width 
  without 
  affecting 
  the 
  

   factor 
  cos 
  a, 
  

  

  (7) 
  Uncertainty 
  in 
  the 
  determination 
  of 
  the 
  current. 
  

  

  The 
  accuracy 
  with 
  which 
  the 
  current 
  can 
  be 
  determined 
  

   is 
  limited 
  by 
  the 
  accuracy 
  with 
  which 
  the 
  E.M.F. 
  of 
  the 
  

   standard 
  cadmium 
  cell 
  is 
  known. 
  The 
  cell 
  used 
  was 
  tested 
  

   at 
  the 
  National 
  Physical 
  Laboratory 
  after 
  the 
  experiment 
  

   was 
  first 
  performed, 
  and 
  the 
  result 
  showed 
  that 
  its 
  E.M.F. 
  

   instead 
  of 
  being 
  1*01987 
  volt 
  at 
  15° 
  C, 
  which 
  was 
  the 
  

   value 
  obtained 
  from 
  the 
  certificate, 
  was 
  1*01880 
  volt. 
  

  

  Cadmium 
  cells 
  show 
  a 
  gradual 
  fall 
  of 
  E.M.F. 
  with 
  time 
  and 
  

   anomalous 
  changes 
  of 
  E.M.F. 
  with 
  changes 
  of 
  temperature. 
  

   It 
  is 
  unsafe 
  to 
  rely 
  on 
  their 
  accuracy 
  to 
  more 
  than 
  1 
  part 
  

   in 
  10,000. 
  This 
  part 
  of 
  the 
  experiment 
  together 
  with 
  the 
  

   timing 
  of 
  the 
  oscillation 
  limits 
  the 
  accuracy 
  to 
  1 
  in 
  10,000. 
  

  

  If 
  it 
  were 
  found 
  necessary 
  the 
  current 
  could 
  be 
  determined 
  

   more 
  accurately, 
  but 
  while 
  the 
  accuracy 
  of 
  determination 
  of 
  

   the 
  oscillation 
  period 
  is 
  so 
  limited 
  there 
  is 
  no 
  point 
  in 
  

   insisting 
  on 
  a 
  greater 
  degree 
  of 
  accuracy 
  in 
  the 
  knowledge 
  

   of 
  the 
  magnitude 
  of 
  the 
  current. 
  

  

  (8) 
  An 
  error 
  in 
  the 
  determination 
  of 
  the 
  number 
  

   of 
  turns 
  per 
  cm. 
  

  

  The 
  method 
  of 
  determining 
  the 
  number 
  of 
  turns 
  per 
  cm. 
  

   has 
  been 
  described 
  already. 
  The 
  method 
  consists 
  of 
  measuring 
  

   the 
  distance 
  between 
  two 
  points 
  enclosing 
  a 
  definite 
  number 
  

   of 
  threads. 
  No 
  error 
  at 
  all 
  can 
  creep 
  in 
  through 
  the 
  deter- 
  

   mination 
  of 
  the 
  number 
  of 
  threads, 
  but 
  the 
  distance 
  is 
  only 
  

   measured 
  to 
  within 
  -005 
  mm. 
  The 
  total 
  distance 
  is 
  26 
  cm., 
  

  

  