Sec. 12.3] 



IONIZATION CHAMBERS 



353 



With the appropriate electrical circuits the rate of charge collection can be 

 read directly in terms of scale divisions per second, or as a null instrument 

 it can be calibrated in terms of millivolts per second required to maintain the 

 fiber at the zero position. The latter method is preferable when the highest 

 accuracy is desired. 



1 rwjry 



"ffl gs 



-$&-=* 



Fig. 98. Schematic diagram of Lindemann electrometer and external circuits. C, ioniza- 

 tion chamber; F, metal-coated quartz fiber with "T" fiber at one end; M, compound 

 microscope; O, octants of electrometer; P, circuit for applying accurately known voltage to 

 octants; S, shorting switch to discharge collecting electrode and fiber; T, torsion fiber. 



C. Vacuum-tube Electrometer. The voltage produced by leakage of the 

 collected charge across a high resistance of the order of 10 11 ohms can be 

 amplified by an electrometer-connected vacuum tube similar to the method 

 described in Sec. 9.11. Several vacuum tubes such as the FP-54 and the 

 Victoreen VX-41 [4,5] have been designed specifically for this purpose. 



This method is applicable to currents as low as 10 -15 amp. The sensitivity 

 can be further extended, to about 10~ 17 amp, if the resistance is removed and 

 the charged is allowed to accumulate. The voltage developed on the grid 

 of the vacuum tube then depends upon the total capacitance of the grid 



