XV. ELECTRONS, NEUTRONS, AND ALPHA PARTICLES 



541 



the order jS rays, protons, a rays. Much detailed information on this 

 subject is available (30,84) • 



The collecting electrode is connected with the measuring instru- 

 ment bj^ a highly insulated wire traveling inside an earthed tube. 

 If the dose rate to be measured is 1 r. per minute and the volume of 

 the ionization chamber is 1 cc, the current to be measured is roughly 

 5 X 10~'^ amp. The current-measuring instrument must therefore 

 be some form of electrometer (85, Ch. IV), electrometer valve, or 

 similar device (86,87), and special attention must be given to the 

 quality of the electrical insulation. 



Ionization 

 chamber 



100 V. 

 battery 



T 3 -way 



J_ X switch 



p^lir-liO/i/ifd. W 

 I — I li — I conaenser 



(air or vacuum) 



Electrometer 

 ~ valve 



Standard 

 cell 



Potentiometer 



Ebonite 

 bushing 



Amber 



Metal 



guard 



ring 



Fig. 15. (a) Diagram of Townsend balance method of measuring ionization 

 current. (6) Representative diagram of an ionization chamber. 



The diagram shows the collecting electrode connected to the con- 

 trol grid of an FP 54 electrometer valve. It is possible to measure 

 the current by noting the rate at which the potential of the grid rises, 

 as judged by the change in anode current. There are several ad- 

 vantages, however, irrespective of the nature of the measuring in- 

 strument, in keeping the collecting electrode near earth potential by 

 some system that balances out the ionization current. The method 

 illustrated in Figure 15a accompHshes this by gradually changing the 

 potential of one plate of the fixed condenser (83) . If such a measur- 

 ing system is employed there should exist a negligible difference of 

 potential between the collecting electrode and the earthed cylinder 

 which enters the base of the ionization chamber, thus imposing a 

 minimum electrical strain on the insulator which supports the collect- 

 ing electrode. The critical insulator is preferably made of amlK-r, not 

 only because amber is an excclleiil insulator but V)ocause its insulating 



