﻿Horizontal 
  Intensity 
  of 
  the 
  Eartlis 
  Magnetic 
  Field. 
  769 
  

  

  and 
  hence 
  we 
  have 
  a 
  possibility 
  of 
  error 
  of! 
  1 
  part 
  in 
  50,000. 
  

   This, 
  being 
  more 
  accurate 
  than 
  the 
  determination 
  of 
  the 
  other 
  

   quantities, 
  can 
  be 
  neglected. 
  

  

  (9) 
  Heating 
  of 
  the 
  coil. 
  

  

  The 
  heating 
  of 
  the 
  coil 
  may 
  affect 
  the 
  results 
  in 
  two 
  ways. 
  

  

  (a) 
  An 
  alteration 
  in 
  resistance 
  of 
  the 
  coil 
  wire, 
  causing 
  a 
  

   corresponding 
  variation 
  in 
  the 
  current 
  flowing 
  through 
  it, 
  

   and 
  therefore 
  of 
  the 
  field 
  produced 
  by 
  the 
  current. 
  

  

  (b) 
  An 
  alteration 
  in 
  moment 
  of 
  the 
  suspended 
  magnet 
  due 
  

   to 
  temperature 
  variation. 
  

  

  (a) 
  it 
  is 
  clear 
  that 
  if 
  the 
  experiment 
  is 
  to 
  be 
  accurate, 
  the 
  

   field 
  produced 
  by 
  the 
  current 
  must 
  be 
  constant 
  to 
  within 
  

   the 
  limits 
  of 
  accuracy 
  of 
  the 
  other 
  determinations. 
  The 
  

   field, 
  and 
  consequently 
  the 
  current, 
  must 
  therefore 
  be 
  con- 
  

   stant 
  to 
  at 
  least 
  1 
  part 
  in 
  10,000. 
  The 
  total 
  resistance 
  in 
  

   the 
  helix 
  circuit 
  is 
  about 
  145 
  ohms, 
  and 
  the 
  resistance 
  of 
  the 
  

   helix 
  is 
  approximately 
  47 
  ohms. 
  The 
  variation 
  must 
  there- 
  

   fore 
  be 
  less 
  than 
  "0145 
  ohm; 
  and 
  as 
  a 
  change 
  in 
  temperature 
  

   of 
  1 
  degree 
  causes 
  a 
  variation 
  of 
  resistance 
  of 
  the 
  helix 
  of 
  

   •172 
  ohm, 
  we 
  see 
  that 
  the 
  maximum 
  change 
  of 
  temperature 
  

   allowable 
  is 
  approximately 
  ^ 
  2 
  °^ 
  a 
  degree. 
  The 
  temperature 
  

   change 
  of 
  the 
  coil 
  is 
  therefore 
  a 
  serious 
  item, 
  and, 
  unless 
  the 
  

   experiment 
  is 
  performed 
  where 
  there 
  are 
  no 
  sudden 
  tempera- 
  

   ture 
  changes, 
  likely 
  to 
  vitiate 
  the 
  results. 
  Being 
  embedded 
  

   in 
  shellac 
  the 
  helix 
  is 
  shielded 
  from 
  sudden 
  temperature 
  

   variations 
  of 
  the 
  room, 
  and 
  the 
  Only 
  factor 
  likely 
  to 
  cause 
  

   ^trouble 
  is 
  the 
  heating 
  of 
  the 
  helix 
  due 
  to 
  the 
  passage 
  of 
  the 
  

   current. 
  However, 
  a 
  thermometer 
  graduated 
  in 
  tenths 
  of 
  a 
  

   degree 
  showed 
  no 
  change 
  whatever 
  during 
  the 
  experiment. 
  

   As 
  the 
  thermometer 
  could 
  be 
  read 
  to 
  ^ 
  of 
  a 
  degree, 
  we 
  see 
  

   that 
  the 
  temperature 
  change 
  does 
  not 
  alter 
  the 
  current 
  to 
  an 
  

   appreciable 
  extent. 
  The 
  thermometer, 
  however, 
  follows 
  the 
  

   temperature 
  of 
  the 
  shellac 
  coating 
  which 
  may 
  lag 
  behind 
  

   the 
  wire 
  a 
  little, 
  so 
  that 
  it 
  is 
  important 
  to 
  calculate 
  roughly 
  

   the 
  temperature 
  change 
  likely 
  to 
  occur. 
  

  

  The 
  current 
  is 
  approximately 
  '014 
  ampere 
  and 
  the 
  re- 
  

   sistance 
  47 
  ohms, 
  hence 
  the 
  heat 
  developed 
  per 
  second 
  is 
  

   *0092 
  calory. 
  This 
  gives 
  31*12 
  calories 
  per 
  hour. 
  

  

  The 
  mass 
  of 
  the 
  wire 
  of 
  the 
  helix 
  is 
  538 
  grins., 
  and 
  as 
  the 
  

   specific 
  heat 
  of 
  copper 
  is 
  '092, 
  we 
  see 
  that 
  49'5 
  calories 
  are 
  

   needed 
  to 
  heat 
  the 
  helix 
  1 
  degree 
  centigrade, 
  assuming 
  that 
  

   no 
  heat 
  is 
  lost 
  by 
  radiation. 
  Thus 
  in 
  one 
  hour, 
  if 
  no 
  heat 
  

   were 
  lost 
  by 
  radiation, 
  the 
  temperature 
  of 
  the 
  coil 
  would 
  

   rise 
  '63 
  degree. 
  The 
  current 
  is 
  only 
  passing 
  for 
  about 
  

  

  