Contact Electricity of Metals, 119 



was effected by putting the divider at zero and bringing- into 

 contact the two pieces of platinum wire. 



Order of Experiment. — The sliding piece of the divider was 

 put to zero, and contact made and broken, and the upper 

 plate raised : then the deflexion of the spot of light was 

 observed. These operations were repeated with the sliding 

 piece at different numbers on the divider scale, until one was 

 found at which the make-break and separation caused no 

 perceptible deflexion. The number thus found on the divider 

 scale was the percentage of the electromotive force of the 

 Daniell cell, which was equal to the contact electric difference 

 of the plates in the Volta-condenser. 



[Addendum, November 23, 1880. — Since the communica- 

 tion of this paper to the British Association, I have found 

 that a dry platinum disc, kept for some time in dry hydrogen 

 gas, and then put into its position in dry atmospheric air in 

 the apparatus for contact electricity, becomes positive to 

 another platinum disc which had not been so treated, but had 

 simply been left undisturbed in the apparatus. The positive 

 quality thus produced by the hydrogen diminishes gradually, 

 and becomes insensible after two or three days.] 



P.S. — On December 24, 1880, one of two platinum plates 

 in the Volta-condenser was taken out ; placed in dried oxygen 

 gas for forty-five minutes ; taken out, carried by hand, and 

 replaced in the Volta-condenser at 12.30 on that day. It was 

 then found to be negative to the platinum plate which had 

 been left undisturbed. The amount of the difference was 

 about *33 of a volt. The plates were left undisturbed for 

 seventeen minutes in the condenser, and were then tested 

 again, and the difference was found to have fallen to *29 of a 

 volt. At noon on the 25th they were again tested, and the 

 difference found to be '18. The differences had been tested 

 from time to time since that day, the plates having been left 

 in the condenser undisturbed in the intervals. The following 

 table shows the whole series of these results : — 



Electric difference between 



surfaces of a platinum plate in 



natural condition, and a platinum 



plate after 45 minutes' exposure 



Time. to dry oxygen gas. 



Dec 21, 12.30 p.m "33 of a volt. 



24, 12.47 p.m '29 „ 



25, noon '18 ,, 



27, noon '116 „ 



28, 11.20 a.m '097 „ 



31, noon *047 ,, 



Jan. 4, 11.0 a.m '042 „ 



11, 11.40 a.m -020 „ 



