MEASUREMENT OF X-RAYS AND RADIUM 



71 



tortion by the use of guard wires a,h,c, . . . , which are spaced uniformly 

 between the guards and high-potential plates (Fig. 15, .4). The wires 

 are maintained at definite, equally graded, potentials by means of a high- 

 resistance potential divider as shown. Essentially, this system prevents 

 the field from "bulging out" at the ends of the chamber. This in turn 

 permits a shortening of the guard plates and allows the surrounding shield 

 to be brought to within 3 or 4 cm. of the electrodes. 



To elecfromefer 



A 



Grenz Rays 



Elec. B 



^/^n ^^oTrf; ^^*i°"^^ Bureau of Standards guarded field standard ionization chamber 

 (6U - 200 kv.) ; 6, guarded field standard ionization chamber for Grenz rays (3 - 90 kv). 



By thus shortening these distances, it is possible to bring the limiting 

 diaphragm D to within 12 to 15 cm. of the measuring electrode. This 

 extends the range of usefulness to considerably lower-voltage X-rays 

 without necessitating large corrections for air absorption between 

 diaphragm P and collector C. 



Standard Measurement of Ionization Currents.— Measurement of the 

 ion current produced in primary standard chambers necessitates appa- 

 ratus for which the calibration is direct as well as permanent. The 

 ordinary galvanometer would be very adaptable, except that sufficiently 

 large ion currents to produce a deflection cannot usually be obtained 

 without introducing other and more serious errors. As a result, most 

 standardization laboratories employ some form of electrostatic measuring 

 device embodying the principle of current compensation (Fig. 16). One 

 circuit. A, consists of an electrometer E, variable condenser C, and a 



