A DETERMINATION OF THE DIURNAL VARIATION OF THE 



RADIOACTIVITY OF THE ATMOSPHERE AT MANILA 



BY THE ACTIVE DEPOSIT METHOD ' 



By 0. H. Blackwood 

 (From the Department of Physics, University of the Philippines) 



FTVE TEXT FIGUBES 



Elster and Geitel ^ first discovered that when a negatively 

 charged wire is exposed in the open air it becomes coated with 

 an active deposit which may be detected by the ionization 

 produced when the wire is placed in the ionization chamber of 

 an electroscope. Bumstead * showed that the collected mass is 

 a mixture of the active deposits of radium and thorium, and 

 it is generally agreed that its sources are the radioactive 

 substances in the earth. Many observers have attempted to de- 

 termine the amount of emanation per cubic meter of the atmos- 

 phere by measuring the ionization produced by the active 

 deposit collected on a given length of wire at a certain voltage 

 and exposed for a definite period of time. Eve * compared the 

 amount of active deposit collected by a wire exposed in the 

 open air with the amount collected by a wire in a closed tank 

 containing the decay products from a known quantity of radium 

 bromide ; and, assuming, on experimental evidence, that the wire 

 charged to 10,000 volts collected all the active deposit from a 

 cylindrical space 40 centimeters in radius concentric with the 

 wire, calculated that the deposit per cubic kilometer at Montreal 

 was in equilibrium with 0.14 to 0.49 gram of radium bromide. 

 This is of the same order of magnitude as the amount after- 

 wards found by Eve ° and others by the charcoal absorption 

 method. 



The active deposit method, owing to its simplicity and to the 

 fact that it is the only one yet devised for the detection of 



' This work has been carried on with the encouragement of Dr. J. R. 

 Wright, who has assisted with many valuable suggestions. 

 ' Phys. Zeitschr. (1901), 2, 590. 

 'Am. Joum. Sci. (1904), IV, 18, 1-11. 

 *Phil. Mag. (1905), 10, 98-112. 

 ''Ibid. (1907), 14, 724. 



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