﻿770 
  Mr. 
  W. 
  A. 
  Jenkins 
  : 
  New 
  Method 
  of 
  Determining 
  the 
  

  

  35 
  minutes, 
  and 
  during 
  this 
  time 
  most 
  of 
  the 
  heat 
  generated, 
  

   i. 
  e. 
  about 
  18 
  calories, 
  will 
  be 
  dissipated. 
  It 
  is 
  not 
  likely, 
  

   therefore, 
  that 
  the 
  temperature 
  would 
  rise 
  y 
  1 
  ^ 
  C. 
  This 
  heat 
  

   generation 
  by 
  the 
  passing 
  of 
  the 
  current 
  could 
  be 
  made 
  of 
  

   still 
  less 
  importance 
  by 
  increasing 
  the 
  external 
  resistance 
  

   and 
  the 
  E.M.F. 
  of 
  the 
  battery 
  sending 
  the 
  current. 
  Thus 
  

   by 
  using 
  a 
  battery 
  of 
  24 
  volts, 
  and 
  consequently 
  an 
  external 
  

   resistance 
  of 
  about 
  1700 
  ohms, 
  a 
  resistance 
  change 
  of 
  not 
  

   greater 
  than 
  *17 
  ohm 
  could 
  be 
  allowed. 
  This 
  admits 
  of 
  a 
  

   rise 
  in 
  temperature 
  of 
  the 
  coil 
  of 
  1 
  degree, 
  and 
  as 
  the 
  

   maximum 
  possible 
  rise, 
  excluding 
  all 
  radiation 
  loss, 
  is, 
  during 
  

   the 
  35 
  minutes 
  during 
  which 
  the 
  current 
  passes, 
  only 
  *37° 
  C, 
  

   we 
  see 
  that 
  the 
  temperature 
  rise 
  would 
  certainly 
  be 
  entirely 
  

   negligible. 
  That 
  it 
  did 
  not 
  introduce 
  an 
  error 
  in 
  the 
  ex- 
  

   periment 
  as 
  it 
  was 
  carried 
  out 
  was 
  confirmed 
  by 
  the 
  fact 
  that 
  

   a 
  determination 
  of 
  the 
  current 
  after, 
  as 
  well 
  as 
  before, 
  the 
  

   timing 
  of 
  the 
  oscillations 
  showed 
  no 
  measurable 
  variation 
  in 
  

   the 
  current. 
  

  

  (b) 
  The 
  temperature 
  coefficient 
  of 
  alteration 
  of 
  moment 
  of 
  a 
  

   magnet 
  is 
  about 
  10~% 
  and 
  as 
  the 
  change 
  of 
  temperature 
  was 
  

   less 
  than 
  1° 
  C, 
  its 
  effect 
  may 
  be 
  neglected. 
  

  

  From 
  the 
  foregoing 
  discussions 
  we 
  see 
  that 
  the 
  most 
  

   important 
  factor 
  is 
  the 
  timing 
  of 
  the 
  oscillations 
  of 
  the 
  

   magnet, 
  for 
  all 
  the 
  other 
  factors 
  can, 
  by 
  suitable 
  methods, 
  be 
  

   made 
  negligible 
  compared 
  with 
  this 
  one. 
  

  

  Consequently, 
  any 
  improvement 
  of 
  apparatus 
  which 
  in- 
  

   creases 
  the 
  period 
  during 
  which 
  the 
  magnet 
  will 
  vibrate, 
  is 
  

   of 
  great 
  value. 
  

  

  » 
  j 
  

  

  Actual 
  Determinations 
  of 
  " 
  H." 
  

  

  For 
  the 
  actual 
  experiment 
  the 
  apparatus 
  was 
  arranged 
  'as 
  

   shown 
  in 
  figs. 
  2 
  and 
  3, 
  the 
  solenoid 
  in 
  fig. 
  2 
  being 
  supported 
  

   as 
  shown 
  in 
  2. 
  The 
  optical 
  system 
  for 
  timing 
  the 
  vibrations 
  

   is 
  not 
  shown 
  in 
  the 
  figures, 
  but 
  was 
  simple. 
  The 
  order 
  of 
  

   performing 
  the 
  experiment 
  was 
  as 
  follows 
  : 
  — 
  

  

  The 
  silk 
  fibre 
  was 
  rid 
  of 
  torsion 
  as 
  previously 
  described, 
  

   and, 
  after 
  inserting 
  the 
  magnet 
  in 
  its 
  holder, 
  the 
  apparatus 
  

   supporting 
  the 
  magnet 
  was 
  placed 
  inside 
  the 
  solenoid, 
  so 
  

   that 
  the 
  magnet 
  was 
  as 
  near 
  as 
  possible 
  to 
  the 
  centre. 
  When 
  

   the 
  solenoid 
  had 
  been 
  adjusted 
  approximately 
  parallel 
  to 
  the 
  

   earth's 
  field, 
  the 
  optical 
  system 
  was 
  arranged 
  so 
  as 
  to 
  give 
  

   an 
  image 
  of 
  the 
  Nernsfc 
  lamp 
  filament 
  coincident 
  with 
  the 
  

   intersection 
  of 
  the 
  cross-wires 
  of 
  the 
  telescope. 
  The 
  final 
  

   adjustment 
  of 
  the 
  position 
  of 
  the 
  helix 
  was 
  then 
  made 
  as 
  

   previously 
  described. 
  The 
  period 
  of 
  vibration 
  of 
  the 
  magnet 
  

   under 
  the 
  influence 
  of 
  the 
  earth's 
  field 
  was 
  then 
  determined 
  

  

  