﻿294 
  Messrs. 
  Spiers, 
  Twyrnan, 
  and 
  Waters 
  on 
  Variations 
  in 
  

  

  Appendix. 
  

  

  Since 
  the 
  above 
  investigation 
  was 
  completed, 
  some 
  experi- 
  

   ments 
  have 
  been 
  made 
  to 
  test 
  whether 
  the 
  lag 
  of 
  E.M.F. 
  

   observed 
  was 
  due 
  io 
  the 
  lag 
  of 
  the 
  temperature 
  of 
  the 
  cell 
  

   behind 
  that 
  of 
  the 
  bath 
  or 
  to 
  diffusion 
  lag. 
  A 
  B.T.-form 
  of 
  

   cell 
  was 
  made 
  up 
  in 
  a 
  tube 
  2 
  cm. 
  diameter, 
  and 
  a 
  thermometer 
  

   inserted 
  in 
  a 
  position 
  similar 
  to 
  that 
  occupied 
  by 
  the 
  zinc. 
  

   The 
  cell 
  was 
  immersed 
  in 
  a 
  water-bath 
  so 
  that 
  the 
  upper 
  

   surface 
  of 
  the 
  paste 
  was 
  about 
  2 
  cm. 
  below 
  the 
  surface 
  of 
  the 
  

   water. 
  The 
  temperature 
  of 
  the 
  bath 
  was 
  then 
  raised 
  uniformly, 
  

   the 
  bath 
  being 
  kept 
  stirred, 
  and 
  the 
  temperature 
  of 
  the 
  cell 
  

   observed 
  from 
  time 
  to 
  time 
  and 
  compared 
  with 
  that 
  of 
  the 
  

   water-bath. 
  The 
  lag 
  of 
  the 
  temperature 
  of 
  the 
  cell 
  behind 
  

   that 
  of 
  the 
  bath 
  was, 
  at 
  a 
  rate 
  of 
  variation 
  of 
  temperature 
  of 
  

   1° 
  in 
  six 
  minutes, 
  0°*15 
  ; 
  at 
  a 
  rate 
  of 
  1° 
  in 
  fifteen 
  minutes 
  

   0°*10 
  ; 
  at 
  a 
  rate 
  of 
  1° 
  in 
  thirty 
  minutes 
  less 
  than 
  o, 
  05. 
  The 
  

   same 
  experiment 
  was 
  tried 
  with 
  the 
  thermometer 
  half 
  in 
  the 
  

   mercury 
  and 
  half 
  in 
  the 
  paste 
  so 
  as 
  to 
  represent 
  the 
  condition 
  

   of 
  things 
  in 
  the 
  H-form. 
  The 
  temperature 
  lags 
  in 
  this 
  

   position 
  were 
  found 
  to 
  be 
  very 
  slightly 
  less 
  than 
  in 
  the 
  

   other. 
  

  

  Experiments 
  were 
  also 
  made 
  in 
  which 
  the 
  cell 
  at 
  a 
  tempera- 
  

   ture 
  of 
  12° 
  C. 
  was 
  placed 
  in 
  the 
  bath 
  at 
  30° 
  C. 
  and 
  the 
  bath 
  

   stirred 
  ; 
  the 
  temperature 
  of 
  the 
  cell 
  was 
  observed 
  at 
  different 
  

   times. 
  The 
  observations 
  showed 
  that 
  at 
  the 
  end 
  of 
  five 
  

   minutes 
  the 
  lag 
  behind 
  the 
  temperature 
  of 
  the 
  bath 
  was 
  o, 
  10, 
  

   at 
  the 
  end 
  of 
  eight 
  minutes 
  practically 
  zero. 
  The 
  tempera- 
  

   tures 
  in 
  these 
  experiments 
  were 
  observed 
  accurate 
  to 
  0°*05 
  C. 
  

  

  A 
  Callendar 
  crystal 
  cell 
  was 
  tested 
  in 
  the 
  same 
  manner 
  as 
  

   the 
  above 
  and 
  gave 
  almost 
  identical 
  results. 
  

  

  It 
  appears 
  from 
  these 
  experiments 
  that 
  about 
  half 
  the 
  lag 
  

   of 
  E.M.F. 
  observed 
  in 
  the 
  H-form 
  of 
  cell 
  is 
  due 
  to 
  the 
  lag 
  of 
  

   the 
  temperature 
  of 
  the 
  cell 
  behind 
  that 
  of 
  the 
  bath, 
  and 
  the 
  

   rest 
  to 
  diffusion 
  lag. 
  

  

  Hence 
  in 
  all 
  experiments 
  with 
  Clark 
  cells, 
  where 
  the 
  

   temperature 
  effects 
  are 
  likely 
  to 
  be 
  serious, 
  it 
  is 
  better 
  to 
  

   discard 
  the 
  B.T.-form 
  of 
  cell 
  and 
  use 
  either 
  Prof. 
  Callendar's 
  

   crystal 
  cell, 
  Prof. 
  Carhart's 
  modification 
  of 
  the 
  B.T.-form, 
  

   or 
  the 
  H-form. 
  The 
  Callendar 
  crystal 
  cell, 
  in 
  which 
  the 
  

   zinc 
  rests 
  in 
  a 
  mass 
  of 
  moist 
  zinc-sulphate 
  crystals, 
  has 
  a 
  very 
  

   small 
  diffusion 
  lag, 
  but 
  has 
  the 
  disadvantage 
  that 
  there 
  is 
  a 
  

   possibility 
  of 
  particles 
  of 
  zinc 
  falling 
  and 
  reaching 
  the 
  

   mercury. 
  This 
  fault 
  is, 
  how 
  r 
  ever, 
  eliminated 
  in 
  his 
  inverted 
  

   type. 
  The 
  Carhart 
  cell, 
  in 
  which 
  the 
  solution 
  is 
  of 
  constant 
  

   strength, 
  also 
  possesses 
  a 
  very 
  small 
  diffusion 
  lag, 
  but 
  it 
  has 
  

  

  \ 
  

  

  