RESEARCHES IN CONTACT ELECTRICITY. 
EXPERIMENT XI. (May 21). 
279 
(Fig. 1, a). 
Time (in minutes). Cu | Fe Cu | Fe, * Cu | Cu, 
(=Cu | Fe+Fe | Fe,). 
0 —63 —63 0 
2 —61 —67 — 4 
7 —58 a0 — 7 
Li. —57 —13'7 —10°7 
47 —55 —75'5 —12°5 
EXPERIMENT XII. (May 9). 
(Fig. 1, ¢). 
Time (in minutes). Al | Zn, Al| Al, 
0 +18 0 
10 — 74 —25°4 
20 —146 —32°6 
30 —16-2 —342 
45 — 24:2 —42°2 
60 — 24 —42 
90 —36 —54 
1350 (observed next morning) —47 —65 
In experiment XI., the second column contains the observed values; the 
third is calculated from it by adding to each number the corresponding number 
from the iron curve; and the numbers of the fourth column are obtained from 
those of the third by subtracting from each the first number, which gives the 
deflection due to polished copper and polished iron. In experiment XII. there 
is no column corresponding to the second column of experiment XI., since the 
zinc surface employed for comparison was kept constant throughout the 
experiment. The corresponding curves for zinc and tin are not represented on 
the diagram because of their great proximity to the iron curve. In the course 
of an hour the change on the zinc was only 6 per cent. greater than the 
corresponding change on the iron ; while in forty minutes there was no appre- 
ciable difference in the changes on the tin and iron surfaces. 
The gradual character of the change here indicated is of special value in the 
present inquiry, as I hope to bring out in the final conclusions to which I have 
been led. Meanwhile it is advisable to give the results of the experiments on 
the temperature-variation of the other metals which I investigated. Though 
not so full and satisfactory as the results for iron, these later researches all 
indicate the same general facts—as may be gathered from the following tables 
for copper hot against iron cold, both surfaces being polished with emery paper 
immediately before contact. 
VOL. XXX. PART I. 2U 
