X. A, 1 Blackwood: Radioactivity of Atmosphere 39 



of a very limited amount of investigation. It was first detected 

 by Simpson/" who made three determinations per day in Lapland 

 for several months and found that there was a maximum in 

 the morning and a minimum in the afternoon. Hess ^^ also 

 found a small variation with a minimum in the evening. 

 Neither of these observers made observations covering the 

 entire twenty-four hours. Dike ^- secured the active deposit by 

 forcing a measured stream of air past charged screens. His 

 mean curve for six series of observations, each set of observation 

 extending over a period of about twenty hours, shows a striking 

 variation. The maximum comes soon after midnight, and is 

 more than twenty times the minimum. 



In the present investigation the meteorological data were 

 secured from the Manila Observatory, which is about 0.1 kilo- 

 meter from the point where the measurements were made. The 

 method was similar to that commonly used in active deposit 

 observations. The testing apparatus was a standard Exner 

 electroscope. The hollow, brass supporting neck was filled with 

 sulphur which supported a central rod and a guard ring, the 

 latter was kept charged to 240 volts by means of a storage 

 battery in order to reduce the natural leak. The upper end of 

 the central rod supported the gold leaf, while the end entering 

 the ionizing chamber was rigidly attached to a wire cage 9 

 centimeters in diameter and 17 centimeters high. The ionizing 

 chamber was of sheet brass, 22 centimeters in diameter and 24 

 centimeters deep. A saturation test of the apparatus showed 

 that for the voltage used in the determinations more than 85 

 per cent of the negative ions reached the charged system. The 

 capacity of the system was about 18 electrostatic units. The 

 motion of the leaf was observed by means of a telescope having 

 a micrometer eyepiece. The electroscope had a sensitiveness of 

 approximately 5 divisions per volt. A bare wire was wound 

 on an iron reel fitting snugly into the ionizing chamber. After 

 the introduction of the reel, the bottom of the chamber was 

 closed, the leaf charged to 240 volts, and the motion of the leaf 

 in divisions per minute recorded as the natural leak. The wire 

 was then stretched horizontally, by means of ebonite rods, at 

 a mean elevation of 3 meters, in the open space north of the 

 physics building. Since no static machine was at first avail- 



"Phil. Trans. Roy. Soc. London (1905), A, 205, 61. 



"^ Sitzungsber. Akad. d. Wisa., math.-nat. Klasse, Wien (1910), 119, 145. 



" Terr. Mag. 1906), 7, 125. 



