220 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 4 



(d, distance between plates; a, area of the plates) and for the cylinder 



capacitor 



2e M (In Rlr) 



- (36) 



k„ 



LE 



For R, r, and L, see Fig. (4-1)5. 



In the region below E s the Gerdien capacitor can thus be used to 

 measure air conductivity, defined by nk, the product of ion concen- 

 tration and ion mobility. 



The ion mobility k can be determined by finding experimentally 

 point S in Fig. (4-1)6; at this point the ion mobility k is equal to 

 k g , so that k can be found from Eqs. (3a or b). A sharp bend at S is 

 present only if the ions in the gas stream have the same charge and 

 mobility. In any other case the characteristic shows several bends 

 from which, in simple cases, the presence of the components (ion 

 spectrum) can be found. 



For summarizing reviews and references, see H. Israel, Arch. tech. ^lessen, 

 V 656-2, August, 1953, and V 656-3, June, 1955; also O. W. Torreson, in 

 J. A. Fleming (ed.), "Physics of the Earth," Chap. 5, McGraw-Hill Book Com- 

 pany, Inc., New York, 1939. An airborne "ion filter" is described by R. Gunn 

 in H. R. Byers (ed.), "Thunderstorm Electricity," Chap. 8, pp. 199ff., The 

 University of Chicago Press, Chicago, 1953. 



b. Excess of Ions of One Polarity. A metal cylinder A packed with 

 fine-grade steel wool is held, electrically insulated, within a grounded- 



metal shield B, as shown in Fig. 

 (4-1)7. The cylinder A is initially 

 discharged. Air containing ions of 

 either polarity is drawn through the 

 steel wool; the ions are discharged 

 in contact with the steel wool and 

 give their charge to the cylinder A. 

 If the concentration of positive and 

 negative charges is equal, the cylin- 

 der A will remain uncharged. How- 

 ever, if more ions of one polarity are 

 present, the cylinder will become 



blower 



Fig. (4-1)7. Excess-ion collector: 

 B, grounded container; A, insulated 

 electrode. 



charged to that polarity, and its voltage will be 



E = 



n'Mt 



where n' is the concentration of the excess charge (i.e., the number 

 per cubic meter of charge of one polarity minus that of the other 



