Noise in Resistances and Electron Streams 



By J. R. PIERCE 



TECHNICALLY correct results in a field are achieved initially in diverse 

 and often confusing and complicated ways. Sometimes, such results 

 are later brought together to give them a more unified form and a sounder 

 basis; such critical summary and exposition is of great value. In quite 

 another way, a worker who uses results established in a field will discover 

 many plausible reasons for believing the results, and he will find eventually 

 that an air of inevitability and ''understanding" pervades the subject. 

 Such "understanding" is not to be confused with the process of rigorous 

 proof carried out step by step, but it can help in organizing and making use 

 of a body of related material. 



The field of ''noise", especially as it affects electron devices and communi- 

 cations in general, is one particularly troublesome to engineers. The sound 

 work on the subject has commonly involved mathematics and especially 

 statistical ideas unfamiliar to many who must deal with the practical prob- 

 lems of noise. In early papers on noise, a great deal of heat was generated 

 in acrimonious controversy between two schools, one of which assigned a 

 uniform noise spectrum to certain noise sources, while the other held this to 

 be inadmissible and got identical answers by more recondite means. 

 Happily, a recent paper by S. O. Rice^ clearly presents both approaches. 

 Rice's paper further provides a fine broad summary of noise problems to- 

 gether with considerable original material. It does not extend far into the 

 field of electronics.2 



The reader who has sufficient time could achieve a profound "understand- 

 ing" of the circuit aspects of noise by reading Rice's paper. The under- 

 standing would involve familiarity with much mathematics useful in itself. 

 To many engineers, however, this might prove a lengthy and painful process. 



The writer proposes to present here a series of plausible arguments for 

 believing certain facts about noise. Both simple circuit considerations and 

 "electronic" effects (as, space charge reduction of noise) are included. The 

 arguments presented are not intended to be original and it is not claimed 

 that they are rigorous; they do seem to be easily understood, and to help in 

 remembering and in using some important practical material. Starting 

 points of the arguments, or "postulates", have been chosen on the basis of 

 familiarity, not simplicity. No effort is made to point out all of the hidden 

 assumptions in the arguments, but a few important ones are indicated. 



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