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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



approximation if the crystal packets are regarded as possessing the properties 

 of macrocrystal graphite. 



While some definite correlations are observed between structure and 

 properties, the intercrystal boundaries in pyrolytic carbon modify its bulk 

 properties for two reasons: First, the effects of the interruption of lattice 

 period at them are presumably greatly accentuated by the anisotropy of 

 the graphitic crystals; and, second, there is an actual chemical contamina- 

 tion at the intercrystal boundaries associated with the presence of peripheral 

 hydrocarbon shells and of sorbed atmospheric constituents, as illustrated 



Table I 



by the comparatively low thermal conductivity and by the changes in 

 resistance and in temperature coefficient of resistance which heating the 

 films in vacuo will produce. 

 Some of the properties of the graphitic carbons are collected in Table I. 



6. Pyrolytic Carbon Film Resistors 



6.1 The Substrate or Core 



The influences of the supporting surface or substrate oh the properties 

 of pyrolytic carbon films are both chemical and physical in nature. Mechan- 

 ical perfection of the carbon films is essential to production of resistors and 

 this perfection is determined in large part by that of the core surface. If 



