﻿Horizontal 
  Intensity 
  of 
  the 
  Earth's 
  Magnetic 
  Field. 
  Ill 
  

  

  by 
  the 
  eye-and-ear 
  method. 
  A 
  rough 
  calculation 
  showed 
  

   that 
  a 
  current 
  of 
  approximately 
  *015 
  ampere 
  would 
  be 
  

   required 
  to 
  give 
  a 
  field 
  of 
  2H, 
  assuming 
  H 
  to 
  be 
  "18. 
  

  

  A 
  small 
  piece 
  of 
  steel 
  was 
  held 
  so 
  as 
  to 
  attract 
  the 
  north 
  

   pole 
  of 
  the 
  magnet 
  to 
  the 
  right, 
  and 
  the 
  current 
  was 
  switched 
  

   on, 
  the 
  direction 
  of 
  the 
  current 
  being 
  such 
  as 
  to 
  give 
  a 
  field 
  

   directly 
  opposite 
  to 
  that 
  of 
  the 
  earth. 
  The 
  magnet 
  was 
  

   attracted 
  to 
  one 
  side 
  a 
  little 
  before 
  switching 
  on 
  the 
  current, 
  

   in 
  order 
  to 
  ensure 
  a 
  knowledge 
  of 
  which 
  direction 
  the 
  magnet 
  

   turned 
  round. 
  The 
  magnet 
  might 
  have 
  turned 
  round 
  either 
  

   way 
  had 
  it 
  been 
  in 
  its 
  normul 
  position, 
  when 
  the 
  current 
  was 
  

   switched 
  on, 
  and 
  it 
  would 
  have 
  been 
  impossible 
  to 
  tell 
  

   which 
  way 
  to 
  turn 
  the 
  suspension-head 
  in 
  order 
  to 
  correct 
  

   for 
  increased 
  torsion. 
  The 
  pulley-wheel 
  was 
  then 
  rotated 
  

   through 
  180° 
  in 
  the 
  direction 
  in 
  which 
  the 
  magnet 
  had 
  

   turned. 
  

  

  The 
  period 
  of 
  oscillation 
  of 
  the 
  magnet 
  in 
  its 
  new 
  field 
  

   was 
  now 
  determined 
  by 
  noting 
  the 
  time 
  for 
  100 
  swings. 
  

   The 
  period 
  was 
  shorter 
  than 
  that 
  under 
  the 
  earth's 
  field, 
  but 
  

   after 
  one 
  or 
  two 
  trials 
  the 
  current 
  was 
  adjusted 
  to 
  give 
  

   approximately 
  the 
  same 
  field 
  as 
  the 
  earth's. 
  The 
  time 
  of 
  

   swing 
  was 
  now 
  found 
  by 
  the 
  eye-and-ear 
  method. 
  The 
  

   period 
  as 
  found 
  by 
  this 
  latter 
  method 
  being 
  a 
  little 
  too 
  long, 
  

   the 
  resistance 
  in 
  the 
  circuit 
  was 
  very 
  slightly 
  diminished, 
  

   and 
  another 
  determination 
  of 
  the 
  time 
  of 
  oscillation 
  made. 
  

   Immediately 
  after 
  each 
  determination 
  of 
  the 
  period 
  of 
  vibra- 
  

   tion, 
  the 
  strength 
  of 
  the 
  current 
  in 
  the 
  solenoid 
  was 
  deter- 
  

   mined 
  as 
  previously 
  described, 
  and 
  the 
  thermometers 
  were 
  

   read. 
  These 
  latter 
  showed 
  no 
  variation 
  throughout 
  the 
  

   experiment. 
  

  

  The 
  experiment 
  was 
  performed 
  at 
  Sheffield, 
  and, 
  owing 
  to 
  

   the 
  trams 
  passing 
  in 
  the 
  neighbourhood 
  of 
  the 
  laboratory 
  

   where 
  the 
  observations 
  were 
  made, 
  an 
  accurate 
  determination 
  

   of 
  the 
  period 
  of 
  oscillation 
  could 
  not 
  be 
  made 
  in 
  the 
  day- 
  

   time. 
  Each 
  time 
  a 
  tram 
  passed 
  the 
  building 
  the 
  magnet 
  

   gave 
  an 
  easily 
  perceptible 
  jerk, 
  and, 
  as 
  the 
  passage 
  of 
  cars 
  

   was 
  frequent, 
  any 
  results 
  obtained 
  in 
  the 
  daytime 
  were 
  

   altogether 
  vitiated. 
  

  

  Two 
  experiments 
  were 
  carried 
  out 
  in 
  the 
  night-time, 
  one 
  

   from 
  12.45 
  a.m. 
  to 
  2 
  a.m. 
  on 
  the 
  morning 
  of 
  January 
  11th, 
  

   1913, 
  and 
  the 
  other 
  during 
  the 
  same 
  hours 
  on 
  April 
  9th, 
  1913. 
  

   The 
  results 
  of 
  these 
  experiments 
  are 
  given 
  below. 
  No 
  tem- 
  

   perature 
  changes 
  of 
  the 
  cadmium 
  cell, 
  standard 
  resistance, 
  

   or 
  solenoid 
  were 
  observable 
  during 
  each 
  experiment. 
  All 
  

   the 
  adjustments 
  were 
  made 
  beforehand, 
  and 
  only 
  the 
  actual 
  

   timing 
  of 
  the 
  oscillations 
  and 
  measurements 
  of 
  the 
  currents 
  

   needed 
  to 
  be 
  performed 
  during 
  the 
  night. 
  

  

  