FLUCTUATION NOISE IN VACUUM TUBES 653 



resistance, floating grid current, and the frequency limits of the 

 ampUfier, equations (8) and (9) may be used to calculate this limiting 

 noise level. For example, if one uses a No. 259B tube with the 

 operating voltages specified for floating grid, an ionization chamber 

 having a capacitance of 15 X 10~^^ farad, and an amplifier having a 

 frequency range from 200 to 5000 cycles per second the limiting 

 noise level is 1 X 10~^ root mean square volt. 



The limiting noise level in a system consisting of a photoelectric 

 cell and thermionic amplifier is determined by thermal agitation in 

 the coupling circuit between the photoelectric cell and amplifier, and by 

 shot noise in the photoelectric current (in circuits where the photo- 

 electric current is very small and the coupling resistance is very high, 

 shot noise from grid current in the vacuum tube becomes appreciable). 

 The noise of thermal agitation may be calculated by means of equation 

 (8) provided rg is now replaced by R, the coupling resistance. If 

 vacuum cells are used, the photoelectric current produces a pure shot 

 noise which can be calculated by equation (9) provided ig is replaced 

 by /, the photoelectric current. In gas filled photocells where collision 

 ionization occurs, the noise is in excess of the value calculated in this 

 manner. ^^ The relative magnitude of shot noise and thermal noise 

 depends on the values of / and R, and by combining equations (8) 

 and (9) it is found that 



F?/tV = elR/IkT = 19AIR, (17) 



where / is expressed in amperes, R in ohms, and T is 300° K. Thus 

 an increase in either I or R will tend to make shot noise exceed thermal 

 noise. This is the desirable condition since it furnishes the largest 

 ratio of signal-to-noise for a given light signal on the photoelectric cell. 



In conclusion I wish to acknowledge my indebtedness to Dr. J. B. 

 Johnson for the helpful criticism he has given during the course of 

 this work. 



13 B. A. Kingsbury, Phys. Rev., 38, 1458 (1931). 



