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BELL SYSTEM TECHNICAL JOURNAL 



A departure from the straight Hne relationship, such as is plotted in 

 Fig. 8, occurs only when the contact is in a relatively high-resistance 

 state due to a very slight compression of the contacting particles. 

 When in this condition contacts are quite unstable and observations 

 upon them erratic, but, in general, the noise originating in them is less 

 than that expected if Eq. (2) held over the entire range of contact 

 resistance values. 



For the study of an aggregate of contacts the carbon cell from a 

 barrier type transmitter was chosen. The structure of this cell (see 

 Insert Fig. 9) is such that one would expect the major portion of the 



30 40 50 100 



CELL RESISTANCE IN OHMS 



300 400 500 



Pig 9^The mean square contact noise voltage in a standard carbon transmitter 

 as a function of the cell resistance. The resistance was varied by changing the 

 amount of carbon filling while the transmitter voltage was held constant at 1.0 volt. 

 Each experimental point represents the average of nine different readings. 



current to be conducted through a small part of the total mass of 

 granules near the bottom of the cell, and carbon added to the top of the 

 cell to act mainly to increase the contact compressions of the conducting 

 contacts. The electrodes of this cell are heavily gold plated, which 

 assures that the resistance observed is almost entirely that of the 

 contacts within the aggregate, a condition which is not fulfilled when 

 carbon electrodes are used. 



