﻿286 
  Messrs. 
  Spiers, 
  Twyman, 
  and 
  Waters 
  on 
  Variations 
  in 
  

  

  together 
  inside 
  a 
  glass 
  vessel 
  in 
  paraffin 
  oil, 
  and 
  during 
  the 
  

   experiments 
  they 
  each 
  underwent 
  precisely 
  the 
  same 
  treat- 
  

   ment. 
  The 
  diameter 
  of 
  each 
  of 
  the 
  two 
  legs 
  of 
  the 
  cell 
  was 
  

   about 
  | 
  inch, 
  and 
  their 
  length 
  about 
  one 
  inch. 
  

  

  The 
  method 
  used 
  for 
  the 
  determination 
  of 
  the 
  E.M.F. 
  was 
  

   as 
  follows. 
  Current 
  from 
  a 
  storage-cell 
  traversed 
  a 
  potentio- 
  

   meter-wire 
  in 
  series 
  with 
  a 
  resistance, 
  the 
  value 
  of 
  which 
  

   was 
  such 
  that 
  the 
  fall 
  of 
  potential 
  along 
  the 
  wire 
  amounted 
  to 
  

   one 
  ten-thousandth 
  of 
  a 
  volt 
  per 
  centimetre-length. 
  The 
  cell 
  

   to 
  be 
  tested 
  was 
  joined 
  up 
  in 
  opposition 
  to 
  the 
  standard, 
  and 
  

   the 
  difference 
  in 
  their 
  E.M.Fs. 
  was 
  balanced 
  against 
  the 
  fall 
  

   of 
  potential 
  along 
  a 
  certain 
  length 
  of 
  the 
  stretched 
  wire. 
  

  

  The 
  standard 
  used 
  in 
  comparison 
  consisted 
  of 
  one 
  or 
  other 
  

   of 
  two 
  cells 
  of 
  the 
  Muirhead 
  type, 
  which 
  had 
  been 
  found 
  to 
  

   have 
  an 
  E.M.F. 
  approximately 
  constant 
  during 
  several 
  years. 
  

   The 
  two 
  cells 
  were 
  in 
  one 
  case, 
  and 
  during 
  the 
  experiments 
  

   were 
  placed 
  in 
  a 
  thermostat 
  in 
  which 
  the 
  temperature 
  could 
  

   be 
  kept 
  constant 
  to 
  0°'01 
  C. 
  The 
  E.M.F. 
  of 
  the 
  standard 
  

   was 
  determined 
  absolutely 
  by 
  balancing 
  it 
  against 
  the 
  P.D. 
  

   between 
  the 
  terminals 
  of 
  a 
  resistance 
  of 
  known 
  value 
  of 
  about 
  

   three 
  ohms, 
  due 
  to 
  a 
  current 
  of 
  about 
  half 
  an 
  ampere 
  passing 
  

   through 
  it. 
  The 
  resistance 
  had 
  been 
  made 
  specially 
  for 
  the 
  

   purpose 
  of 
  manganin 
  wire, 
  wound 
  upon 
  an 
  ebonite 
  frame 
  and 
  

   immersed 
  in 
  paraffin 
  oil. 
  It 
  had 
  been 
  previously 
  aged 
  by 
  

   heating, 
  and 
  had 
  assumed 
  a 
  steady 
  value, 
  as 
  shown 
  by 
  com- 
  

   parison 
  with 
  standard 
  manganin 
  coils. 
  The 
  current 
  was 
  

   measured 
  by 
  a 
  Kelvin 
  centiampere 
  balance, 
  the 
  constant 
  of 
  

   which 
  had 
  been 
  frequently 
  checked 
  by 
  means 
  of 
  a 
  silver 
  

   voltameter. 
  By 
  this 
  means 
  the 
  E.M.F. 
  could 
  be 
  determined 
  

   absolutely, 
  correct 
  to 
  0*0001 
  volt. 
  

  

  The 
  thermostat 
  consisted 
  of 
  an 
  inner 
  air-space, 
  in 
  which 
  

   was 
  placed 
  the 
  standard, 
  surrounded 
  by 
  an 
  outer 
  water-bath 
  

   which 
  was 
  heated 
  by 
  a 
  gas-jet, 
  the 
  gas 
  being 
  automatically 
  

   cut 
  off 
  when 
  the 
  temperature 
  rose 
  to 
  a 
  given 
  value, 
  and 
  being 
  

   allowed 
  to 
  come 
  on 
  again 
  as 
  soon 
  as 
  the 
  temperature 
  had 
  

   fallen 
  below 
  that 
  value. 
  

  

  In 
  fig. 
  1, 
  A 
  B 
  C 
  are 
  zinc 
  vessels 
  : 
  A 
  is 
  the 
  air-bath, 
  con- 
  

   taining 
  the 
  standard 
  resting 
  on 
  cottonwool 
  ; 
  B 
  the 
  surrounding 
  

   water-bath, 
  closed 
  at 
  the 
  top 
  and 
  containing 
  the 
  thermo- 
  

   meter 
  D 
  ; 
  C 
  the 
  enclosing 
  jacket, 
  between 
  which 
  and 
  B 
  the 
  

   hot 
  air 
  from 
  the 
  gas-jet 
  passes, 
  heating 
  the 
  water-jacket 
  on 
  

   its 
  way. 
  D 
  is 
  an 
  alcohol 
  thermometer 
  with 
  a 
  mercury- 
  

   index 
  F, 
  which, 
  on 
  the 
  expansion 
  of 
  the 
  alcohol, 
  is 
  pushed 
  up 
  

   the 
  narrow 
  tube 
  L, 
  and 
  electrically 
  connects 
  the 
  two 
  platinum 
  

   wires 
  /i 
  and/ 
  2 
  » 
  J 
  * 
  s 
  the 
  gas-jet 
  for 
  heating 
  the 
  water-bath 
  ; 
  

   K 
  a 
  baffle-plate 
  over 
  the 
  gas-jet, 
  to 
  prevent 
  the 
  flame 
  heating 
  

  

  