304 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



of the ambient, the hermetical enclosure is filled with an inert gas, oxygen- 

 free nitrogen or helium normally being employed. Hydrogen is not used 

 because it is not wholly "inert": Carbon films sealed in this gas increase 

 in resistance with time, as if there were a tendency for them to revert to 

 the hydrocarbons from which they were produced. 



Resistors sealed in vacuo or hehum exhibit a small initial aging, of the 

 order of 100 parts per milUon (PPM) in resistance value, which can be 

 completely eliminated by cycling them between —80 deg and 120 deg C. 

 After this thermal cycling, the resistors do not change in resistance with 

 time or further cycling. Measurements over more than seven years show 

 the resistance to be stable to at least ±50 PPM and the temperature co- 

 efficient of resistance to about 0.2 PPM deg C~^, within the limits of meas- 

 uring accuracy^. For resistance values ranging from 100,000 ohms to tens 

 of megohms, these stabilities are far greater than can be obtained in any 

 other type of present-day resistor. 



Certain applications of these precision hermetically enclosed units have 

 required that all resistors in a given network possess temperature coeffi- 

 cients alike to within 1.0 PPM deg C~^ As illustrated in Fig. 14, the tem- 

 perature coefficient of resistance of the film, a, depends on the film thickness, 

 and hence all such "tracking" resistors are produced from a constant film 

 thickness, different resistance values being obtained by the techniques of 

 adjustment which have been described. The value of a for the films employed 

 is 300 ± 35 PPM deg C~^ While this value is from 3 to 6 times larger in 

 absolute magnitude than that for wirewound units, its statistical variation 

 for these resistors is no greater. This statistical variation in a, however, 

 makes it necessary to measure each resistor, if groups with values of a 

 differing by no more than 1.0 PPM deg C~^ are required. 



Precision hermetically sealed resistors are very sensitive to faulty seals; 

 and failure to reproduce resistance values at a constant reference tempera- 

 ture after temperature change is a criterion of the effectiveness of the seal, 

 resistance changes greater than 15 PPM in absolute value being sufficiently 

 large to be significant in this respect, if this change occurs in a relatively 

 short time. The resistance of the film attains a stable value in a given gaseous 

 environment, but if this environment changes only very slightly in com- 

 position or pressure it is necessary to restabilize once more by thermal 

 cycling. If the composition changes with time, as in the case of a leaking 

 envelope, stabilization to these accuracies is impossible. 



The sensitivity of resistance value of carbon films to their gaseous en- 

 vironments would seem to be associated with adsorption equilibria, and there 

 are data to show that adsorption of certain materials is more deleterious 

 than that of others. There is evidence, moreover, that adsorption may not 

 only change the number or mobility of the electrons in carbon, but that it 



