Force  in  the  Contact  of  Metals.  217 
Exp.  18.  Intensity  =  tan  41°  50'. 
Deviations. 
23-70 
24-70 
23-73 
Mean     .     .     24-04 
From  these  three  experiments  we  obtain  j3  =  0*485 1,  and  from 
Exp.  16 «  =  31*40 
17 a  =  30-65 
18 a  =  31-65 
Mean     .     .  3123 
Some  months  after  the  above  experiments,  the  copper-alumi- 
nium combination  was  again  introduced  into  the  apparatus,  and 
I  made  the  two  following  : — 
Exp.  19.  Intensity  =  tan 38°  15'.|  Exp.20.  Intensity  =  tan  20°  55'. 
Deviations. 
2103 
21-30 
Mean  .  .  21-17 
Deviations. 
11-82 
10-13 
Mean  .  .  10-98 
If  we  calculate  the  last  two  experiments  in  the  usual  manner, 
we  obtain  £  =  0-3182,  and  «  =  29*29. 
Taking  the  mean  of  the  four  values  of  a  for  the  copper-alumi- 
nium combination,  we  obtain  the  final  result 
«  =  30*77. 
The  passage  of  the  positive  current  from  the  aluminium  to  the 
copper  produced  a  cooling  at  the  surface  of  union. 
Three  values  of  a  have  been  obtained  for  each  of  the  above- 
mentioned  combinations.  The  concordance  of  these  values 
within  the  limits  of  possible  errors  of  observation  proves  that  the 
formulae  of  calculation  made  use  of  answer  their  purpose.  An- 
other series  of  observations  on  the  iron-copper  combination,  for 
which  I  used  two  copper  cylinders  not  silvered  (and  which  is  not 
inserted  here,  because  it  is  not  comparable  with  the  others),  fur- 
nished five  values  of  a,  likewise  agreeing  with  each  other  within 
the  limits  of  errors  of  observation.  The  same  result  was  given 
by  the  experiments  in  which  the  observations  took  place  at  in- 
tervals of  fifteen  minutes. 
Copper-  Gold. 
Exp. 21.  Intensity  =  tan 36°40/. 
Deviations. 
10-90 
10-88 
Mean  .  .  10*89 
Exp. 22.  Intensity  =  tan 40°45'. 
Deviations. 
1300 
11-98 
Mean   .  .  12*49 
