﻿Volta 
  Electricity 
  of 
  Metals. 
  427 
  

  

  § 
  75. 
  I 
  shall 
  now 
  give 
  a 
  specimen 
  experiment. 
  The 
  two 
  

   standard 
  gold 
  plates 
  gave 
  

  

  -•02 
  v., 
  

  

  the 
  minus 
  sign 
  indicating 
  that 
  the 
  upper 
  plate 
  is 
  negative. 
  I 
  

   now 
  heated 
  the 
  upper 
  ; 
  when 
  " 
  tepid," 
  L 
  e., 
  about 
  35° 
  C, 
  it 
  

   gave 
  with 
  the 
  cold 
  plate 
  

  

  -•045 
  v. 
  

  

  It 
  was 
  next 
  heated 
  further 
  until 
  " 
  very 
  hot 
  " 
  (57° 
  C.) 
  and 
  

   gave 
  

  

  -•06 
  v. 
  

  

  When 
  it 
  had 
  cooled 
  down 
  to 
  " 
  tepid 
  " 
  (35° 
  C), 
  it 
  gave 
  

  

  -•04 
  v. 
  

   When 
  " 
  cool 
  " 
  (28° 
  C.), 
  

  

  -•03 
  v. 
  

  

  Some 
  hours 
  later, 
  when 
  both 
  plates 
  were 
  cold, 
  they 
  gave 
  as 
  

   at 
  first 
  

  

  -•02 
  v. 
  

  

  Hence 
  the 
  potential 
  of 
  gold 
  which 
  has 
  been 
  washed 
  with 
  

   alcohol 
  and 
  allowed 
  to 
  dry 
  falls 
  temporarily 
  about 
  *0016 
  v. 
  

   per 
  degree 
  centigrade 
  rise 
  of 
  temperature. 
  

  

  § 
  76. 
  Most 
  of 
  the 
  temperature 
  experiments 
  on 
  copper 
  were 
  

   made 
  with 
  the 
  hollow 
  plate 
  previously 
  mentioned, 
  which 
  was 
  

   filled 
  with 
  hot 
  water 
  in 
  which 
  the 
  bulb 
  of 
  a 
  thermometer 
  was 
  

   placed. 
  In 
  many 
  of 
  the 
  experiments 
  on 
  zinc, 
  and 
  also 
  on 
  

   aluminium, 
  a 
  thermo-electric 
  arrangement 
  was 
  used, 
  and 
  the 
  

   results 
  obtained 
  with 
  it 
  do 
  not 
  differ 
  materially 
  from 
  those 
  

   obtained 
  by 
  the 
  above 
  method 
  (§69). 
  These 
  and 
  other 
  

   details 
  are 
  noted 
  on 
  the 
  diagram. 
  

  

  § 
  77. 
  The 
  diagram 
  on 
  p. 
  428 
  gives 
  the 
  temperature-variations 
  

   of 
  all 
  the 
  substances 
  studied. 
  The 
  curves 
  in 
  it 
  are 
  plotted 
  by 
  

   taking 
  the 
  results 
  for 
  each 
  metal 
  of 
  those 
  experiments 
  which 
  

   are 
  most 
  free 
  from 
  all 
  complication 
  or 
  cause 
  of 
  doubt. 
  If 
  the 
  

   curves 
  be 
  prolonged 
  in 
  the 
  direction 
  of 
  lower 
  temperature 
  

   they 
  appear 
  to 
  meet 
  somewhere 
  below 
  -200° 
  C, 
  and 
  prob- 
  

   ably 
  asymptotically 
  to 
  the 
  line 
  representing 
  *4 
  v. 
  positive 
  to 
  

   standard 
  plate 
  at 
  16° 
  C. 
  Within 
  their 
  range 
  they 
  show 
  

   contact-potential-differences 
  diminishing 
  with 
  lowered 
  tem- 
  

   perature. 
  The 
  only 
  apparent 
  exceptions 
  are 
  clean 
  copper, 
  

   and 
  silver 
  coated 
  with 
  glass 
  ; 
  but 
  both 
  their 
  curves 
  are 
  

   distinctly 
  bent 
  between 
  16° 
  C. 
  and 
  50° 
  C, 
  so 
  that 
  probably 
  

   they 
  are 
  directed 
  towards 
  the 
  same 
  point 
  as 
  the 
  others 
  at 
  

   ower 
  temperatures. 
  Thus 
  it 
  appears 
  that 
  at 
  about 
  — 
  200° 
  C. 
  

   the 
  contact- 
  potential-differences 
  of 
  metals 
  may 
  vanish, 
  and 
  

  

  