652 



BELL SYSTEM TECHNICAL JOURNAL 



10 



z 



uli to 



10' 



10' 



10 



10 



10*= 10-^ 10* 



INPUT RESISTANCE IN OHMS 



10- 



10° 



Fig. 5 — Thermal noise level as a function of input resistance and frequency range. 



and 15,000 cycles, it is found that the equivalent thermal noise input 

 is 0.9 X 10-^ volt, while the shot noise input is 1.4 X 10"^ volt. The 

 total noise is the square root of the sum of the squares of these values 

 or 1.7 X 10~^ volt. This agrees with the measured value of 1,4 

 X 10~^ volt within an error of 20 per cent, which is as accurate as the 

 determination of the grid currents. These equations may also be 

 used to calculate the noise originating in the grid circuit when external 

 resistance or capacitance is connected between grid and cathode, pro- 

 vided Yg and c are now calculated from the internal and external 

 impedances in parallel. 



A common method of detecting corpuscular or electromagnetic 

 radiation makes use of an ionization chamber and linear amplifier. 

 In this circuit the control grid in the first tube of the amplifier is con- 

 nected to the collecting electrode of the ionization chamber and both 

 allowed to float at equilibrium potential.^* The shot and thermal 

 noise in this grid circuit sets a limit to the measurement of extremely 

 weak radiation. Knowing the value of input capacitance, input 



18 H. Greinachcr, Zeits. f. Physik, 36, 364 (1926). 



