﻿the 
  Electromotive 
  Force 
  of 
  Clark 
  Cells 
  with 
  Temperature. 
  291 
  

  

  to 
  occur. 
  In 
  this 
  method 
  we 
  measure 
  the 
  difference 
  of 
  the 
  

   E.M.Fs. 
  of 
  the 
  cells 
  on 
  the 
  stretched 
  wire, 
  and 
  not 
  the 
  

   actual 
  E.M.F. 
  of 
  the 
  cell 
  under 
  test, 
  consequently 
  considerable 
  

   variations 
  of 
  the 
  current 
  have 
  very 
  little 
  effect 
  on 
  the 
  result. 
  

  

  The 
  cell 
  under 
  test 
  was 
  placed 
  at 
  T, 
  and 
  by 
  joining 
  the 
  mer- 
  

   cury 
  cups 
  b, 
  c 
  and 
  #, 
  y, 
  this 
  could 
  be 
  joined 
  up 
  with 
  its 
  E.M.F. 
  

   in 
  opposition 
  to 
  that 
  of 
  the 
  standard 
  ; 
  and 
  then 
  by 
  moving 
  the 
  

   contact 
  P, 
  we 
  could 
  find 
  a 
  point 
  such 
  that 
  the 
  potential- 
  

   difference 
  between 
  P 
  and 
  C 
  equalled 
  the 
  difference 
  in 
  E.M.F. 
  

   between 
  the 
  cell 
  to 
  be 
  tested 
  and 
  the 
  standard. 
  If 
  the 
  

   difference 
  between 
  the 
  E.M.Fs, 
  was 
  positive, 
  then 
  the 
  point 
  

   P 
  lay 
  on 
  one 
  side 
  of 
  the 
  point 
  C, 
  while 
  if 
  the 
  difference 
  was 
  

   negative, 
  it 
  lay 
  on 
  the 
  other 
  side. 
  

  

  To 
  find 
  how 
  the 
  changes 
  in 
  E.M.F. 
  lagged 
  behind 
  those 
  of 
  

   the 
  temperature, 
  it 
  was 
  found 
  best 
  to 
  subject 
  the 
  cells 
  to 
  a 
  

   cycle 
  of 
  temperature, 
  the 
  temperature 
  being 
  first 
  raised 
  at 
  a 
  

   uniform 
  rate 
  through 
  a 
  certain 
  number 
  of 
  degrees, 
  and 
  then 
  

   cooled 
  again 
  at 
  the 
  same 
  uniform 
  rate 
  to 
  the 
  original 
  tempera- 
  

   ture. 
  Supposing 
  no 
  lag 
  to 
  exist, 
  the 
  curve 
  connecting 
  

   temperature 
  and 
  E.M.F. 
  would 
  be 
  the 
  same 
  for 
  both 
  rising 
  and 
  

   falling 
  temperatures. 
  But 
  if 
  there 
  is 
  a 
  lag 
  of 
  short 
  duration, 
  

   i. 
  e. 
  not 
  cumulative, 
  then, 
  provided 
  that 
  the 
  temperature 
  varies 
  

   at 
  the 
  same 
  rate 
  throughout, 
  two 
  distinct 
  curves, 
  the 
  second 
  

   parallel 
  to 
  the 
  first 
  for 
  the 
  greater 
  part 
  of 
  its 
  length, 
  will 
  

   result, 
  and 
  will 
  enclose 
  an 
  area 
  between 
  them. 
  If 
  there 
  is 
  

   any 
  lag 
  of 
  a 
  semi-permanent 
  character, 
  it 
  will 
  be 
  shown 
  by 
  the 
  

   curve 
  not 
  returning 
  to 
  its 
  original 
  starting-point 
  till 
  after 
  a 
  

   considerable 
  time. 
  

  

  When 
  we 
  proceeded 
  to 
  take 
  the 
  cells 
  through 
  a 
  cycle 
  of 
  

   temperature, 
  we 
  first 
  of 
  all 
  determined 
  absolutely 
  the 
  E.M.F. 
  

   of 
  the 
  standard, 
  which 
  had 
  been 
  kept 
  at 
  a 
  constant 
  tempera- 
  

   ture 
  for 
  some 
  days 
  in 
  the 
  thermostat. 
  The 
  current 
  for 
  heating 
  

   the 
  cell-bath 
  was 
  then 
  turned 
  on, 
  the 
  required 
  value 
  of 
  the 
  

   current 
  to 
  give 
  a 
  certain 
  rate 
  of 
  rise 
  of 
  temperature 
  having 
  

   been 
  previously 
  determined 
  by 
  experiment. 
  The 
  current 
  in 
  

   the 
  stretched 
  wire 
  was 
  then 
  adjusted 
  to 
  its 
  proper 
  value, 
  and 
  

   the 
  readings 
  of 
  the 
  E.M.Fs. 
  of 
  the 
  cells 
  were 
  taken 
  every 
  few 
  

   minutes. 
  The 
  temperature 
  of 
  the 
  cells 
  was 
  observed 
  every 
  

   five 
  minutes 
  and 
  a 
  curve 
  plotted 
  connecting 
  temperature 
  and 
  

   time. 
  The 
  heating 
  curve 
  between 
  time 
  and 
  temperature 
  

   always 
  gave 
  practically 
  a 
  straight 
  line, 
  without 
  readjustment 
  

   of 
  the 
  current. 
  When 
  we 
  started 
  to 
  cool 
  our 
  bath, 
  we 
  turned 
  

   on 
  the 
  iced- 
  water 
  circulation 
  full 
  for 
  the 
  first 
  few 
  minutes, 
  to 
  

   get 
  a 
  sharp 
  bend 
  in 
  the 
  temperature 
  and 
  time 
  curve, 
  after- 
  

   wards 
  it 
  was 
  made 
  to 
  go 
  much 
  slower, 
  and 
  the 
  flow 
  required 
  

   readjusting 
  from 
  time 
  to 
  time 
  to 
  keep 
  the 
  curve 
  straight. 
  

  

  