﻿1875.] 
  

  

  of 
  VerdeVs 
  Constant 
  in 
  Absolute 
  Units* 
  

  

  505 
  

  

  units 
  for 
  a 
  standard 
  substance. 
  Distilled 
  water 
  was 
  used, 
  and 
  the 
  mag- 
  

   netic 
  force 
  was 
  produced 
  by 
  means 
  of 
  an 
  electric 
  current 
  in 
  a 
  helix, 
  as 
  

   the 
  magnetism 
  of 
  iron 
  magnets 
  is 
  an 
  undetermined 
  function 
  of 
  the 
  shape 
  

   and 
  nature 
  of 
  the 
  iron 
  core. 
  

  

  The 
  strength 
  of 
  the 
  helix 
  was 
  determined 
  by 
  comparing 
  the 
  magnetic 
  

   force 
  at 
  a 
  series 
  of 
  seven 
  equidistant 
  points 
  along 
  its 
  axis 
  in 
  terms 
  of 
  

   that 
  at 
  the 
  centre 
  of 
  the 
  great 
  dynamometer 
  of 
  the 
  British 
  Association, 
  

   whose 
  power 
  is 
  known 
  in 
  absolute 
  measure. 
  

  

  The 
  intensities 
  were 
  compared 
  by 
  varying 
  currents 
  sent 
  opposite 
  ways 
  

   through 
  each, 
  till 
  the 
  action 
  on 
  a 
  small 
  magnet 
  at 
  their 
  common 
  centre 
  

  

  The 
  intensity 
  at 
  each 
  of 
  a 
  series 
  of 
  points 
  being 
  known 
  for 
  a 
  given 
  current, 
  

   the 
  difference 
  of 
  magnetic 
  potential 
  at 
  the 
  two 
  ends 
  for 
  that 
  current 
  was 
  

   obtained 
  by 
  integrating 
  with 
  respect 
  to^ 
  the 
  length 
  between 
  limits 
  corre- 
  

  

  any 
  point. 
  

  

  The 
  difference 
  of 
  magnetic 
  potential 
  at 
  the 
  ends 
  for 
  a 
  certain 
  current 
  

   being 
  known, 
  the 
  strength, 
  1ST, 
  of 
  the 
  helix 
  (which 
  is 
  the 
  ratio 
  of 
  this 
  

   difference 
  to 
  the 
  current, 
  or 
  the 
  difference 
  of 
  magnetic 
  potential 
  which 
  

   would 
  be 
  due 
  to 
  a 
  unit 
  current) 
  is 
  known, 
  and 
  is 
  a 
  number, 
  because 
  current 
  

   and 
  magnetic 
  potential 
  are 
  of 
  the 
  same 
  dimensions. 
  

  

  In 
  the 
  helix 
  used, 
  which 
  was 
  about 
  26*34 
  centims. 
  in 
  length 
  and 
  13 
  

   centims. 
  in 
  diameter, 
  we 
  had 
  

  

  The 
  absolute 
  value 
  of 
  the 
  degrees 
  of 
  a 
  tangent 
  galvanometer 
  was 
  also 
  

   determined 
  by 
  placing 
  it 
  under 
  the 
  dynamometer. 
  

  

  To 
  determine 
  the 
  rotation 
  of 
  the 
  plane 
  of 
  polarization, 
  a 
  Nicol's 
  prism, 
  

   set 
  in 
  a 
  circle, 
  was 
  used, 
  and 
  the 
  light 
  was 
  polarized 
  by 
  means 
  of 
  a 
  prism 
  

   invented 
  by 
  Professor 
  Jellett, 
  and 
  described 
  by 
  him 
  in 
  vol. 
  xxv. 
  of 
  the 
  

   Transactions 
  of 
  the 
  Royal 
  Irish 
  Academy. 
  

  

  It 
  was 
  constructed 
  of 
  Iceland 
  spar, 
  and 
  its 
  field 
  of 
  vision 
  consisted 
  of 
  

   a 
  circle 
  divided 
  by 
  a 
  line, 
  the 
  light 
  of 
  one 
  half 
  of 
  which 
  was 
  polarized 
  in 
  a 
  

   certain 
  plane, 
  and 
  the 
  light 
  of 
  the 
  other 
  half 
  in 
  a 
  plane 
  inclined 
  at 
  about 
  

   2° 
  to 
  that 
  of 
  the 
  first. 
  The 
  intermediate 
  position 
  of 
  the 
  Mcol, 
  when 
  

   the 
  whole 
  field 
  was 
  equally 
  dark, 
  could 
  be 
  determined 
  with 
  some 
  

   accuracy. 
  

  

  The 
  water 
  was 
  contained 
  in 
  a 
  tube 
  with 
  glass 
  ends, 
  of 
  the 
  same 
  length 
  

   as 
  the 
  helix, 
  and 
  placed 
  with 
  it. 
  The 
  polarized 
  ray 
  was 
  sent 
  through 
  it, 
  

  

  was 
  nil. 
  

  

  ]Sr= 
  10752. 
  

  

  