﻿the 
  Electromotive 
  Force 
  of 
  Clark 
  Cells 
  with 
  Temperature. 
  293 
  

  

  The 
  probable 
  reason 
  for 
  the 
  superiority 
  of 
  the 
  H-form 
  

   over 
  the 
  ordinary 
  form 
  is 
  that 
  when 
  the 
  changes 
  of 
  tern- 
  

   perature 
  produce 
  variations 
  in 
  the 
  density 
  of 
  the 
  zinc 
  sulphate 
  

   solution, 
  the 
  zinc 
  amalgam, 
  being 
  at 
  the 
  bottom, 
  is 
  always 
  in 
  

   contact 
  with 
  a 
  saturated 
  solution, 
  while 
  in 
  the 
  B.T.-form 
  we 
  

   have 
  the 
  solution 
  in 
  contact 
  with 
  the 
  mercury 
  saturated, 
  but 
  

   that 
  in 
  contact 
  with 
  zinc 
  will 
  not 
  be 
  saturated 
  till 
  the 
  whole 
  

   of 
  the 
  paste 
  has 
  attained 
  its 
  equilibrium 
  condition 
  and 
  again 
  

   become 
  saturated 
  with 
  zinc 
  sulphate 
  at 
  the 
  new 
  temperature. 
  

  

  Thus 
  in 
  general 
  it 
  appears 
  that 
  in 
  the 
  H-form 
  of 
  Clark 
  cell 
  

   the 
  lag 
  is 
  much 
  less 
  than 
  in 
  the 
  B.T.-form, 
  and 
  that 
  under 
  

   ordinary 
  conditions, 
  when 
  the 
  rate 
  of 
  variation 
  of 
  temperature 
  

   is 
  not 
  greater 
  than 
  l°in 
  half 
  an 
  hour, 
  by 
  applying 
  the 
  ordinary 
  

   temperature-corrections 
  we 
  can 
  obtain 
  a 
  result 
  accurate 
  to 
  less 
  

   than 
  a 
  ten-thousandth 
  of 
  a 
  volt. 
  If 
  we 
  wish 
  to 
  apply 
  a 
  

   temperature-correction 
  for 
  a 
  greater 
  rate 
  of 
  variation 
  than 
  

   this, 
  we 
  must 
  either 
  find 
  the 
  temperature-coefficient 
  for 
  varia- 
  

   tions 
  at 
  that 
  rate 
  from 
  curves 
  such 
  as 
  those 
  given 
  here, 
  or 
  

   else 
  slightly 
  diminish 
  the 
  ordinary 
  coefficient. 
  Subsequent 
  

   experiments 
  showed 
  that 
  if 
  an 
  H-cell 
  were 
  placed 
  in 
  a 
  water- 
  

   bath, 
  which 
  was 
  heated 
  through 
  3° 
  or 
  4° 
  as 
  quickly 
  as 
  possible, 
  

   then 
  the 
  E.M.F. 
  of 
  the 
  cell 
  would 
  reach 
  an 
  approximately 
  

   steady 
  value 
  as 
  soon 
  as 
  the 
  temperature 
  of 
  the 
  bath, 
  and 
  by 
  

   taking 
  the 
  temperature 
  of 
  the 
  cell 
  as 
  being 
  that 
  of 
  the 
  bath, 
  

   we 
  could 
  apply 
  the 
  temperature 
  correction 
  accurate 
  to 
  a 
  ten- 
  

   thousandth 
  of 
  a 
  volt. 
  

  

  The 
  two 
  Muirhead 
  cells 
  used 
  as 
  standards 
  in 
  the 
  above 
  

   experiments 
  were 
  also 
  taken 
  through 
  a 
  single 
  cycle 
  at 
  a 
  rate 
  

   of 
  variation 
  of 
  temperature 
  of 
  1° 
  in 
  fifteen 
  minutes. 
  From 
  

   the 
  curves 
  (fig. 
  8) 
  it 
  will 
  be 
  seen 
  that 
  one 
  of 
  the 
  cells 
  is 
  but 
  

   slightly 
  inferior 
  as 
  regards 
  lag 
  to 
  the 
  H-form, 
  while 
  the 
  other 
  

   comes 
  about 
  midway 
  between 
  the 
  H 
  and 
  the 
  B.T. 
  forms. 
  

   These 
  two 
  cells 
  have 
  been 
  recently 
  dismounted, 
  and 
  it 
  was 
  

   found 
  that 
  they 
  were 
  almost 
  dried 
  up 
  and 
  that 
  the 
  zinc 
  

   sulphate 
  had 
  crystallized 
  out 
  over 
  the 
  top 
  of 
  the 
  cells. 
  Thev 
  

   had 
  thus 
  practically 
  become 
  examples 
  of 
  Prof. 
  Calendar's 
  

   crystal 
  cell*, 
  and 
  this 
  probably 
  accounts 
  for 
  their 
  small 
  

   lag. 
  The 
  cell 
  which 
  appeared 
  the 
  drier 
  of 
  the 
  two 
  had 
  the 
  

   larger 
  lag. 
  

  

  Our 
  best 
  thanks 
  are 
  due 
  to 
  Prof. 
  Ayrton 
  and 
  Mr. 
  T. 
  Mather 
  

   for 
  their 
  many 
  valuable 
  suggestions 
  and 
  kind 
  help., 
  and 
  to 
  

   Mr. 
  Cooper 
  for 
  the 
  loan 
  of 
  the 
  cells 
  tested. 
  

  

  * 
  Proc. 
  Roy. 
  Sac. 
  October 
  1897, 
  

  

  