RESEARCHES IN CONTACT ELECTRICITY. 283 
Kou rauscu, any change in the difference of potential could be measured in 
terms of a Daniell cell. The results obtained fully corroborated the former 
conclusions, as a glance at the following table will show. The first column, 
headed 6 V, gives the variation of potential for a rise of temperature of 1° C. 
expressed in terms of a Daniell cell; and the second column, headed p, 
indicates the range of probable error in the estimate which was deduced as the 
mean of several distinct experiments. 
éV p 
Zine, , : . —'0028 +0003 
Iron, , ‘ ; . —'002 +0004 
Copper, . : ; . —'001 +0002 
Tin; ‘ ; . —'001 +0002 
It may be noticed that of these zinc gave the most regular results. In 
deducing these numbers it was assumed that the variation varied directly with 
the temperature throughout the range of 60° C. Thus, polished zinc at 20° C. 
gives with polished zinc at 80° C. a difference of potential equal to ‘168 of a 
Daniell cell—the hotter surface being, of course, the negative surface. 
Many definite results were also obtained for the time-variation for aluminium, 
zinc, iron, and copper. The representative curves were in all cases similar to 
those shown in fig. 1. This being understood, the following numbers indicate 
the difference of potential between the polished metal surface and the same 
surface after twenty-four hours’ standing. 
Aluminium, . ; ; . 38 
Zine, ‘ ; : Z ' ee : ; 
Iron, ; 4 (in terms of 1 Daniell cell.) 
Copper, . ; ; : ; 086 
It was found, however, that different days’ experiments gave somewhat varying 
results—the atmospheric conditions as to temperature, humidity, &c., having 
probably some effect. Indications were also obtained in the course of experi- 
ment that this time-variation depended upon the more arbitrary conditions 
under which the varying surface was allowed to vary; whether, for example, 
it was freely exposed to the air, or was left close to the opposed surface; 
whether it was the negative or positive element in the condenser, and such like. 
Where so many possible factors enter, however, it is extremely difficult to 
draw any sure conclusions. 
VOL. XXX. PART I. DEX 
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