SECTION III, 1902. ret] Trans. R. S. C. 
VIL— Eecited Radioactivity Produced from Atmospheric Air. 
By $. J. ALLAN, M.Sc. 
Demonstrator in Physics, McGill University. 
(Communicated by Prof. Rutherford, and read May 27, 1902.) 
Elster and Geitel have shown,’ that when a negatively charged 
body such as a wire, is exposed to the air for some time it becomes 
temporarily radioactive, that is, it has the power of discharging elec- 
tricity from a charged body, and that this radioactivity decayed with 
time. They also showed that this radioactivity could be dissolved 
off by dilute acid and after being evaporated down to dryness, the 
residue was still radioactive. 
The radioactivity in its properties is thus similar to that produced 
by the radioactive compounds, thorium and radium. 
This paper gives a brief account of experiments made by the 
author during the past winter at McGill University. In most of the 
work hitherto done on this subject the experimenters have used the 
gold leaf electroscope method to observe the rate of leak of the 
charged body. 
The author was fortunate enough to be able to make use of the 
very sensitive electrometer used by Prof. Rutherford in his work on 
radium and thorium. Whilst the method hitherto used has been 
slow and did not allow of sufficient readings being taken, the electro- 
meter gave very rapid and accurate readings. Some of the experi- 
ments have already been described in a paper by Prof. Rutherford 
and the author.? 
The method used was as follows: — The wire on which to collect 
the radioactivity was stretched between two insulated supports. The 
negative terminal of a Wimshurst machine was then attached to the 
wire, the positive terminal of the machine being earthed. The wire 
was thus kept at a certain potential above earth. To ensure the 
potential always being about the same during the run, an adjustable 
spark gap was introduced between the wire and earth. The distance 
between the knobs was arranged so that it would never quite spark 
across, thus ensuring the potential never rising above a certain value. 
The spark gap was calibrated and we could always tell approximately 
the voltage the wire was at. The Wimshurst machine was driven by 
an electric motor. 

1 Phys. Zeit., 1901. 
2 Phys. Zeit., 1902. 
