296 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 9 



depending upon the circuit values used, and it may be supplied from batteries 

 or an electronic supply. If the latter, it should be extremely well filtered 

 and very highly regulated in order to avoid drift and short period transients. 

 Extreme care must be taken in the assembly of such circuits to eliminate 

 contact potentials, acid soldering flux, short leakage paths, etc., and all 



# u 



— !i — l 



A 



A 



A 



A 



A 

 -O 



Fig. 72. Typical example of current amplifiers that are now used for measuring steady 

 currents as small as 10~ 15 amp. The negative feedback, d-c amplifier shown here was 

 developed by Nier especially for use with mass spectrometers. The power supply voltage 

 for the plates is 225 volts, and the voltage at A is adjusted for a filament current of 150 

 milliamperes. Precision wire-wound resistors are indicated by PWW. [Redrawn from 

 A. O. Nier, Rev. Sci. Instruments, 18, 398 (1947).] 



Rl = 4 X 10 10 ohms, IRC type MG-6 



R2 = 5,000 ohms, PWW 



R3 = 20,000 ohms, PWW 



i?4 = 40 ohms, PWW 



R5 = shunt to limit filament current in Fl 



to 20 ma 

 R6 = 12 steps of 13 ohms each, PWW 

 Rl = 200,000 ohm potentiometer 

 R$ = 20,000 ohm potentiometer 

 R9 = 200,000 ohms, PWW 



RIO RU, RU = 1 megohm, 1 watt 



R12 = 7,500 ohms, 10 watts 



i?14 = 35,000 ohms, PWW 



R\5 = 10,000 ohms, PWW 



it 16 = 15,000 ohms, PWW 



CI = 100 MMfd, variable condenser 



C2 = 0.02 MMfd, 400 volt condenser 



VI = VX-41 (Victoreen Instrument Co.) 



V2 = 12SJ7 



V3 = VR75 



74 = 12J5 



resistive elements should be wound from zero temperature-coefficient wire. 

 The use of long leads from the output to a galvanometer or recording device 

 is permissible. 



The nonlinear characteristics of the electrometer tube over the range of 

 currents (10~ 14 to 10 -10 amp) which is ordinarily measured must be accurately 

 compensated. In manual or static measurements the electrometer-gal- 



