418 Proceedings of Royal Society of Edinburgh. [sess. 
We have used two different methods to measure the potential 
difference between two mutually insulated metals when the air 
between them is rendered conductive by the presence of uranium. 
The more convenient method is to take uranium as one of the mutu- 
ally insulated metals. To do this we fixed a metallic disc, 3 cms. 
diameter, to the insulated terminal of a quadrant electrometer. 
Opposite this metallic disc, and separated from it by air, we placed 
a disc of uranium, 5*5 cms. diameter, connected to the other ter- 
minal of the electrometer. With this arrangement a steady read- 
ing, the metallic zero, was obtained when the quadrants of the 
electrometer were in metallic connection. After contact between 
the quadrants was broken at the electrometer a deviation from the 
metallic zero took place gradually to a point, the uranium-conduc- 
tance-zero we shall call it, depending on the volta difference between 
the two opposed surfaces of metals, more or less tarnished as they 
generally are. On the other hand, if the insulated metal had a 
charge given to it of such an amount as to cause the electrometer 
reading to deviate from the metallic zero beyond the uranium- 
conductance-zero, the reading quickly fell to this conductance-zero 
and there remained steady. When no charge was given to the 
insulated metal the steady conductance-zero was reached in about 
half a minute. The following table gives 
the potential differences 
found in this way : — 
Metals. 
Potential Difference. 
Volts. 
Polished aluminium (1) immediately after 
being polished, .... 
- 1*13 
Polished aluminium (1), next day, 
-1-90 
Polished aluminium (2), 
-U00 
Amalgamated zinc, 
-0-80 
Polished zinc, .... 
-0*71 
Unpolished zinc, 
-0-55 
Polished lead, .... 
-0*54 
Tinfoil, ..... 
- 0*49 
Unpolished aluminium (1), 
-0-41 
Polished copper, 
-0*17 
Unpolished copper, 
+ 0-07 
Silver coin, .... 
+ 0-05 
Carbon, ..... 
+ 0-20 
Oxidised copper (a), 
+ 0-42 
Oxidised copper (6), 
+ 0-90 
With a third specimen of oxidised copper a potential difference 
