
352 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
carriers, possessing a similar property, were present in the atmo- 
sphere. For this purpose a cylinder of wire-netting, charged nega- 
tively to 600 volts, was exposed for several hours in the open air. 
This was then removed and quickly placed im a large bell-jar, inside 
which was placed an electroscope to detect the rate of discharge. 
They found that the rate of discharge was increased to a slight 
extent. In order to multiply the effect, a wire of about 20 metres 
long was exposed at some height from the ground, and was kept 
charged to a high potential by connecting it to the negative 
terminal of an influence machine. After exposure for some hours, 
this wire was removed and placed inside the dissipation vessel. 
The rate of discharge was found to be increased many times by the 
presence of the wire. No increase was observed if the wire had 
been charged positively instead of negatively. The results also 
showed that the radio-active matter could be removed from the 
wire in the same way as from a wire made active by exposure in the 
presence of the thorium emanation. A piece of leather moistened 
with ammonia was rubbed over the active wire. On testing the 
leather it was found to be strongly radio-active. If a long wire 
were used, the amount of activity obtained on the leather was 
comparable to that possessed by a gram of uranium oxide. 
The activity produced on the wire was not permanent, but 
disappeared to a large extent in the course of a few hours. The 
amount of activity produced on a wire of given size, exposed under 
similar conditions, was independent of the material of the wire. 
Lead, iron and copper wires gave about equal effects. 
The amount of activity obtained was greatly increased by ex- 
posing a negatively charged wire in a mass of air which had been 
undisturbed for a long time. Experiments were made in the great 
cave of Wolfenbiittel, and a very large amount of activity was 
observed. By transferring the activity to a piece of leather it 
was found! that the rays could appreciably light up a screen of 
barium platinocyanide in the dark. The rays also darkened a 
photographic plate through a piece of aluminium 0°1 mm. in 
thickness. 
These remarkable experiments show that the excited radio- 
activity obtained from the atmosphere is very similar in character 
1 Phys. Zeit. 3, p. 76, 1901. 
