ABSTRACTS 



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ATMOSPHERIC ELECTRICITY.— Orz certain new -atmospheric-electric 

 instruments and methods. W. F. G. Swann. Terr. Mag., 19: 171- 

 185. 1914. 



The first instrument described is a modification of the Ebert ion- 

 counter, designed with a view to securing greatly increased sensitive- 

 ness. There are two concentric cyUnders, as in the Ebert apparatus, 

 but instead of the central one being connected to an electroscope read- 

 ing up to 200 volts, as in that instrument, it is connected to the fiber 

 of a single-fiber electroscope of the Einthoven type adjusted to a sen- 

 sitivity of about 20 or 30 divisions per volt. The potential of the fiber 

 is never allowed to depart far from zero potential and the necessary 

 field is obtained by insulating and chargmg the outer cylinder to about 

 200 volts. On releasing the fiber from earth it, of course, starts to 

 move, and the rate of movement can be noted. In order to prevent 

 the charge on the outer cylinder affecting the number of ions coimng 

 to the apparatus, it is shielded by another cylinder which is earthed. 

 In order to insure that the charge on the -upper edge of the shielding 

 cylinder does not rob the air of ions which should go to the central rod, 

 a special attachment is made to the latter which renders this impossible. 



The second portion of the paper describes a modification of Elster 

 and Geitel's apparatus for determining the radio-active content of the 

 atmosphere. Instead of the collecting wire being charged to —2000 

 volts below the potential of the earth, as in Elster and Geitel's method, 

 it is charged to —2000 volts below the potential which the air in its 

 vicinity would have if it were absent. In this way the charge on the 

 wire, which is shown to determine the amount of active deposit col- 

 lected, is independent of the potential gradient. A more fundamental 

 unit of activity is suggested than that used by Elster and Geitel. It is 



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