72 



BIOLOGICAL EFFECTS OF RADIATION 



battery of known potential F (11). As the charge +Q from the ioniza- 

 tion chamber / flows onto the insulated system for a time t, the condenser 

 is changed through a capacity (C2 - d) inducing an opposite charge -Q 

 on the system, so as to always maintain the electrometer at zero deflec- 

 tion. The current is then given by Q/t = v{C2 - Ci)/t. An alternative 

 method (45) is to keep C constant and vary v (Fig. 15, B) whence the 

 current i = Q/t = €{¥2 - Vi)/L Since the accuracy of these methods 

 depends only on measurements of capacitance, voltage, and time, it is 

 obvious that a high precision is obtainable. Circuits A and B are well 

 adapted to a simple calibration when E is such an instrument as a string 

 or quadrant electrometer. Where ^ is a vacuum-tube electrometer, the 



A I 



4 - 



^ 



B 



X 



^ - 



/www. 



i-HltliliK. 



A 



I I /vww 



4 f 



I. 



Fig. 16. — Standard measuring systems for very small currents. 



cahbration becomes less direct and more uncertain, depending upon 

 the constancy of the vacuum-tube characteristics. 



A third type of measuring circuit (Fig. 16, C) employs a radioactive 

 compensator RA for neutraUzing the ion current (13). A small amount 

 of radium is sealed in a tube and placed in the chamber RA, to which 

 may be applied a known variable potential. The relationship of the 

 radioactive ionization current to V is determined by separate calibration. 

 A convenient modification consists in applying a fixed potential to a 

 variable radioactive source of ionization (25) so designed that the capacity 

 of the system remains constant during any change in the compensator. 



In a fourth method (Fig. 16, D) a known potential V is applied across 

 a high resistance R of the order of IQi" ohms, the current balance being 

 noted by the electrometer E. Knowing R, the current is given directly 

 by Ohm's law / = V/R. For accurate measurements it is necessary 

 that R be calibrated frequently or until its constancy is assured. 



