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BELL SYSTEM TECHNICAL JOURNAL 



mechanical means arc provided for roiling this chamber at any desired 

 speed; if all of the air and sorbed gases are removed from the carbon 

 chamber and pure nitrogen is substituted; if the mean temperature is 

 kept constant within 2° C ; and if means are provided for measuring the 

 resistance of the granules when at rest by observing the voltage 

 across the two electrodes while current is allowed to flow for a period 

 less than 1/200 of a second, it is found that the resistance (for most 

 samples of carbon) may be di-tmnined within a fraction of one [ler 



f^iJ 



M^k^^'ipidm^ 





Fig. 1 



cent. If, however, the button is rotated (even as slowly as possible) 

 and then i)rought to rest, the resistance may difi'er several ohms from 

 its first value. If a large number of observations arc made aflcr 

 this fashion, we may e.\pect to find for certain samples of carbon a 

 set of values such as given in Fig. 1. The 270 observations of re- 

 sistance reproduced 'n this figure were made on a sample of carbon 

 at 1^ volts under conditions quite similar to those outlined above. 

 The observed variation is from approximately 215 to 270 ohms. 

 The upper cur\c is that of the resistance \s. the serial number of 

 the readings. There is no apparent trend in the change of resistance 

 from one reading to another. The lower curve in this figure shows 

 the frequency histogram of the results. Attention is directed to the 



