Sec. 4-1] 



ELECTRICAL TRA XSlJl VERS 



219 



volume flow velocity M in cubic meters per second. If the voltage E 

 is sufficiently high so that all ions are deposited on the electrodes, the 

 current is 



I=nq31 (I) 



The voltage-current characteristic of the counter is shown in Fig. 

 (4-1)6. The law expressed by Eq. (1) is valid only in the saturation 

 region, i.e., at voltages above E s . In this region, the system can be 

 used for the determination of nq, the concentration of charges, or, 

 if q is known, the concentration of ions (n + or n~). Positive and nega- 

 tive charges are counted separately by applying either a negative or a 

 2R 



Fig. (4-1)5. Gerdien ion collector. 



Voltage E 



Fig. (4-1)6. Voltage-current 

 characteristics of the ion col- 

 lector, used for the determina- 

 tion of the ion mobility. 



positive potential to the insulated electrode C. The ion currents are 

 frequently too small to be measured by the voltage-drop method 

 indicated in Fig. (4-1)5; higher sensitivity is obtained by electrom- 

 eter methods. The electrometer is inserted between the points A 

 and G; it is charged initially and should not be allowed to discharge to 

 a value below E s . 



In the region below E s not all ions are drawn to the electrode; the 

 current then depends upon the ion mobility k (i.e., the velocity of 

 the ion, in meters/second, if exposed to a field of 1 volt/m). In this 

 region the current is 



k 



r 



man — 

 k n 



(2) 



where k g is the "threshold mobility," i.e., the mobility of those ions 

 which are just driven to the electrodes. Ions with a mobility smaller 

 than k g are not deposited at the electrodes. The threshold mobility 

 is a constant of the apparatus, it depends upon the counter geometry 

 and the applied voltage and is, for the parallel-plate capacitor with 

 homogeneous flow throughout the volume between the plates, 



e Md 

 kg ~ aE 



(3a) 



