II. M. Dadourian — Atmospheric Radio-activity . 339 









Exposures 



Range of 



Half-value 





necessary to 



a-particle 



period 



% 



make I =-991, 



Radium Em 4-33 cm 



3*8 days 



2-11 X10" 6 sec- 1 



25-3 days 



" A 4-83 



3 min. 



3-85 X10~ 3 " 



0-33 hrs. 



" B (no a-rays^ 



1 26 " 



4-44 XlO- 4 " 



2-9 " 



" C 7-06 cm 



19 " 



6-08 XlO- 4 " 



2-1 " 



Thorium Em 5*55 



54 sec. 



1-28 XlO" 2 " 



6-1 " 



• " , A (rayless) 



11 hrs. 



1-75 XlO" 5 " 



V3-1 " 



" B 5-0 



1 hr. 



1-93 XlO" 4 " 



6-6 " 



" C 8-6 



few sec * 







2. Open air. — A copper wire J mm in diameter and about 

 100 meters in length was strung between two buildings of 

 the Scientific School, at a height of about seven meters from 

 the ground. The wire was insulated and connected to the 

 negative electrode of a Wimshurst machine, the positive pole 

 of which was earthed. A spark gap of 2 mm was maintained 

 between the electrodes of the machine. After exposures of 

 four days the potential difference was removed from the wire 

 and the latter was placed in a testing vessel. The ionization 

 was measured by means of a Dolezalek electrometer connected 

 to the testing vessel. The initial total activity was over one 

 hundred times as large as that produced by the spontaneous 

 ionization in the apparatus. 



From 20 to 30 one-hundred ths of the initial total ionization 

 was found to be due to the thorium products and the balance 

 due to radium products. In other words, the initial ionization 

 produced by the thorium products was from 25 to 40 one- 

 hundredths of that due to radium products. This is much less 

 than that observed by Blancf in Rome. He found that of the 

 initial total activity of a wire, which was exposed to the open' 

 air in Rome for three days, from 50 to 70 per cent was due to 

 the thorium products. Both his observations and those of the 

 writer are based on arbitrary units of ionization, therefore no 

 conclusion can be drawn as to the relative amounts of radium 

 and thorium emanations present in a unit volume of the atmos- 

 pheres at Rome and at New Haven. But it is certain that 

 the ratio of thorium emanation to radium emanation is greater 

 in the Roman than in the New Haven atmosphere. 



3. We can make use of the above results to find the ratio of 

 the amount of the thorium emanation to the radium emanation 

 present in a unit volume of the air at New Haven. But in 

 order to do this it is necessary to find a relation between the 

 amount of emanation per unit volume and the activity of a 

 wire exposed to the emanation. 



*Hakn, Phil, 

 f Blanc, Phil. 



, xi, 793, 1906. 

 , xiii, 378, 1907. 



