224 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 4 



environment; i.e., the ions and electrons in the atmosphere will cause 

 a charge or discharge of the probe until its potential is equal to the 

 space potential at the place of the probe. 



This exchange of charges between the probe and its surroundings 

 is a slow process and may lead to erroneous readings if the field and 

 the space potential vary with time or if the amount of charges 

 carried from or to the electrode S becomes comparable to that leaking 

 off through the insulator. The situation is evident from Fig. (4-3)2, 

 where the charge-discharge path between the probe S and the sur- 

 rounding potential P s is indicated by an equivalent resistor R e , the 

 capacitance between the insulated probe and ground by C, and the 

 insulation resistance by R a . Only for R a > R e is the voltage E 

 across the capacitor equal to the space potential E s . A number of 

 systems have been developed which permit a faster exchange of 

 charges between the probe S and its environment and lead to a reduc- 

 tion of the equivalent resistance R e to a value between 10 9 and 10 12 

 ohms. 1 



A reduction of the magnitude of R e has the further advantage of 

 reducing the time constant of the probe system, which, for R n > R e , 

 is proportional to R e C. Since C (including the electrometer connected 

 to it) is of the order of 100 /u/uF, the time constant of a collector is 

 between 0.1 and 100 sec. 



water-drop collector. A container with water is connected 

 with the probe electrode. The water is either permitted to drip from 

 the container or is forced by compressed air through one or several 

 nozzles, so that it leaves the container in the form of a spray. Each 

 droplet carries with it a certain amount of charge (the magnitude and 

 polarity determined by electrostatic induction), until the potential 

 of the probe is equal to the space potential. An airborne water 

 collector is described by Gunn. 2 



Drawbacks of the system are the need for replenishment of the 

 water, the insulation difficulty of the water container if located in- 

 doors and connected to the outdoor probe, or the danger of freezing. 



IONIZATION COLLECTORS, POINT-DISCHARGE COLLECTOR. The Col- 

 lector is connected with a fine point. The electric field strength at 

 the point is high so that ionization occurs. The discharge current 

 from the point to the surrounding field is 



i = a{F 2 - ilf 2 ) 



1 H. Israel, Arch. tech. Messen, V 656-3, June, 1953. 



2 R. Gunn in H. R. Byers (ed.), "Thunderstorm Electricity," Chap. 8, pp. 

 199ff., The University of Chicago Press, Chicago, 1953. 



