272 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



cients of resistance larger than are desirable and considerable attention has 

 consequently been given to the use of thin alloy films of lower temperature 

 coefficient. But even though certain such alloys have specific resistances con- 

 siderably greater than pure metals, their resistivities still are so low that 

 exceptionaUy thin films must be employed in order to obtain high values of 

 resistance. The specific resistance and its temperature coefficient for these 

 thin alloy films may depart radically from those characteristic of the bulk 

 metal: The apparent specific resistance may be larger than that of the bulk 

 metal by orders of magnitude, and the temperature coefficient of resistance 

 is often negative and of large magnitude. Associated with these differences, 

 which can be ascribed to departures in structure from that of the bulk 

 metal, there is a decreased stability of the electrical characteristics of the 

 films. Because of this, very thin metal films cannot be employed, and film 

 resistances* of 500 to 1,000 ohms seem to represent the present usable upper 

 Umit. However, within these limits metal alloy films can, with care, be made 

 to yield stable resistors. Other "metals" such as germanium and silicon have 

 also been investigated; and, while films of very high film resistances have bsen 

 produced from them, the temperature coefficients of resistance are large and 

 contact potentials at the film terminals are most troublesome. 



Study of metal film resistors has thus led in the present day, as earlier, 

 to the necessity of employing non-metallic materials of high specific resist- 

 ance in the fabrication of resistors. Of the many materials studied over the 

 years, carbon has proved to be the most generally satisfactory, both be- 

 cause it possesses a relatively high specific resistance and because it can read- 

 ily be produced in film form. 



One widely employed method of producing carbon film resistors involves 

 the application of a carbon-laden liquid "paint" to the surface of a suitable 

 insulating substrate and subsequent curing of the paint film to impart the 

 requisite conductivity and mechanical stability. The carbon particles em- 

 ployed may be of graphite, petroleum coke, coal, channel or furnace carbon 

 blacks, or of combinations of these. Matrices of greatly varied types have 

 been employed, ranging from organic materials such as phenolic, urea-formal- 

 dehyde, and silicone resins to low melting inorganic glasses. The film resist- 

 ance of such films is markedly dependent on the nature of the paint vehicle, 

 on the type of carbon pigment, and on the curing conditions. 



It is characteristic of carbon-pigmented films that their resistances repre- 

 sent the integrated contributions of a large number of single contacts be- 

 tween carbon particles embedded in an essentially insulating matrix. Such 

 contacts, while not "loose" in the sense of similar contacts in the telephone 



* The "film resistance", or the resistance for a square of the film measured between 

 opposite edges, depends only on the resistivity of the material and the thickness of the 

 film. 



