Radioactivity of the Atmosphere. 141 
the air at any time was found to be strongly influenced by 
weather conditions. A cold, clear, windy day gave the 
largest amount and a warm dull day the least. 
In the experiments described in the present paper the 
radioactivity was obtained from a closed room which gave a 
constant amount from day to day. 
The electrical method of measuring the radiations has been 
used throughout the experiments. The electrometer employed 
was of the ordinary Thomson quadrant type, fitted with a 
needle of light construction, which was kept connected to 
one pole of a battery of 300 volts. A reflecting mirror and 
a millimetre-scale indicated the movement of the needle. 
Each scale-division corresponded to 34, of a volt P.D. of the 
quadrants. A small quantity of uranium served to standar-. 
dize the readings of the instrument. 
Increase of Excited Activity with Teme. 
It has been shown in a previous paper * that the excited 
activity derived from the air decays according to an exponen-. 
tial law, the rate of decay being given by the equation 
= —At 
i 21, e=™, 
where [ is the excited activity at any time ¢, I) the maximum 
value, and Xa constant. If the excited activity produced on 
a negatively-charged wire is due to a constant supply of 
positive radioactive carriers whose activity decays according 
to the above equation, then the intensity of the activity L 
after an exposure for a time ¢ will be given by 
I, = I) (1—e-™), 
where I, is the maximum value and «a the same constant as. 
before. 
The following experiment was made to verify this view :— 
About 60 feet of copper wire was suspended in a large attic 
and kept charged to a constant negative potential of about 
20,000 volts, by means of a Wimshurst machine driven by 
an electric motor. In parallel with the charged wire was 
arranged an adjustable spark-gap to regulate the constancy 
of the potential of the wire. After acertain time of exposure 
the wire was taken down and wound lengthwise on an iron 
frame. This frame was placed inside a cylindrical vessel of 
zine and connected to one pair of quadrants of the electro-. 
meter, the other pair being earthed. 
The zine cylinder was connected to one pole of the battery 
and the other pole was to earth. Between the iron frame 
* Rutherford and Allan, Joc. cit. 
