MEASUREMENT OF X-KAYS AND RADIUM 



77 



is then indicated by the decrease in the deflection of the fiber. The 

 scale is calibrated directly in roentgens. The constancy of the electrical 

 calibration of the instrument is readily checked by inserting the ioniza- 

 tion chamber completely discharged into the fully charged electroscope 

 and noting the deflection after the charge has been divided between the 

 two parts of the system. 



Condenser for regu/crHon 

 of sensifiv/fy-^ 



Tonlzah'on 



Fig. 20. — Integrating vacuum-tube roentgenometer. (Mutscheller.) 



Thimble chambers having vacuum indicators are usually of the "relay 

 type," that is, a certain small quantity of radiation causes a relay to 

 operate and move an indicator one step, these steps being successively 

 added up to give the total indication on a dial (31, 40). Such instru- 

 ments require no scale correction and are frequently referred to as inte- 

 grating meters. 



Figure 20 shows one form of such instrument manufactured in this 

 country. Several other similar types are made abroad (21). The 

 ionization current flowing through the chamber is caused to accumulate 

 upon one electrode of a grid-glow tube until a definite potential is devel- 

 oped upon which an ionization discharge takes place in the tube. This 

 discharge initiates the flow of a separate current through, the tube which 

 in turn operates the relay and a counting or recording mechanism. 



