28 ROYAL SOCIETY OF CANADA 
which carry a negative charge. In describing his experiments upon 
the amount of excited radioactivity communicated in air at different 
pressures to metallic conductors in the presence of thorium oxide, he 
states that, in air at very low pressures, he found a negatively charged 
rod became but feebly radioactive, while the sides of the vessel, on 
the contrary, which contained the oxide, the negatively charged rod 
and the attenuated air, exhibited a strongly excited radioactivity. 
As the thorium oxide was enclosed in a paper envelope, and so 
was not in direct contact with the sides of the vessel, this experiment 
indicated that the emanation which was the cause of the excited 
radioactivity could be concentrated under certain circumstances, on 
a positively charged body, although the effect was found generally 
associated with one which carried a negative charge. 
This experiment led the author to examine the effect discovered 
by Elster and Geitel more closely, and, among other points, to see 
if radioactivity was not excited, even feebly, on a positively charged 
conductor exposed in free air. 
A series of experiments was made during a period extending over 
some months with conductors exposed to the air in a large room in 
the Physical Laboratory, as well as with some exposed to the free 
air outside, and it was found that not only did conductors which were 
negatively charged become radioactive, but at times those which were 
positively charged also acquired a marked, though feeble, radioac- 
tivity. The following paper contains an account of the experiments 
dealing with radioactivity excited in positively charged bodies, together 
with a few observations on some of the causes which affect the intens- 
ity of the activity imparted to conductors exposed with a negative 
charge. 
(IT) APPARATUS. 
In conducting these experiments the radioactivity was detected 
by means of an electroscope similar to that devised by C. T. R. 
Wilson’ for the purpose of investigating spontaneous ionization in 
air and other gases. The instrument is shown in Fig. I. 
A thin strip of brass, A, was suspended vertically in a small 
metallic box, B, by means of an insulating bead of sulphur, C. To 
this strip there was attached a gold leaf whose deflexions, observed 
with a micrometer microscope through a mica window in the appa- 
ratus, gave a measure of the potential of the brass strip. 
A brass rod, D, which supported the measuring system, passed 
through an ebonite plug, #, and was connected to one of the terminals 

1 C. T. R. Wilson, Proc. Roy. Soc., Vol. 68, p. 154. 
