330 CARNEGIE INSTITUTION OF WASHINGTON. 



The formula enables more accurate expressions to be deduced for the flow of 

 electricity to the charged system in certain atmospheric electric instruments 

 than have generally been used. For example, the theory of the Gerdien con- 

 ductivity apparatus has heretofore been deduced on the assumption that the 

 instrument may be treated as part of two concentric cylinders of infinite 

 length, and moreover neglects the effect of the supporting rod of the central 

 cylinder. The present formula enables a more accurate expression to be 

 immediately obtained, including the effect of the rod. The true expression 

 leads to values of the conductivity 20 per cent different from those calculated 

 on the usual formula, and in some instruments with thick supporting rods 

 the difference has amounted to as much as 40 per cent. The theory further 

 shows that uniformity of velocity over a cross-section is not necessary, and it 

 also leads to a very ready method of determining the limits of applicability of 

 the apparatus. 



The theory of the collection of active deposits by a charged wire is discussed 

 in relation to the above theory, certain serious sources of uncertainty are 

 pointed out, and more exact formulae are deduced. 



On certain new atmospheric-electric instruments and methods. W. F. G. Swann. Terr. 

 Mag., vol. 19, pp. 171-185 (1914). 



The first instrument described is a modification of the Ebert ion counter. 

 In the usual form of ion counter the central cylinder of the instrument is con- 

 nected to an electroscope reading up to about 200 volts, and the outer cylinder 

 is earthed. The rate of movement of the fibers of the electroscope when the 

 air flows through the instrument under these conditions is extremely slow, and 

 many minutes are required to obtain a single satisfactory observation. More- 

 over, during the observation the quantity being measured varies and this niay 

 give rise to serious trouble; for instance, in the determination of the specific 

 velocity of the ions by Ebert's method, in which it is necessary for the ionic 

 density to remain constant during two experiments, the variation of the ionic 

 density is often so great as to apparently completely ^vipe out the quantity 

 measured, or even render it apparently negative. In the instrument described 

 the central cylinder is connected to the fiber of a single-fiber electroscope of 

 the Einthoven type adjusted to a sensitivity 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 charging 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 coming 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 employment of a 

 sensitive electroscope rendered possible by the above scheme enables the ionic 

 density to be determined easily ten or twenty times as rapidly as by the 

 ordinary apparatus. The apparatus may also be used in conjunction with the 

 usual subsidiary attachment for determining the specific velocities of the ions. 



The second portion of the paper describes a convenient arrangement which 

 may be employed for determining the radioactive content of the atmosphere 

 by the Elster and Geitel method. It is shown that the rate of collection of 

 active material is proportional to the charge on the collecting wire, and in 

 order to render this a definite quantity depending only on the potential of the 

 charging battery, and independent of the potential gradient, the collecting 

 wire. A, is placed on the same equi-potential surface as another Avire, B, from 

 which it is insulated. B is kept at the potential " of the air around it by two 



