PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 301 



erature coefficient of resistance. A typical helixing machine is shown in 

 Fig. 18. 



Choice of film resistance and helix pitch is made to yield a resistor blank 

 slightly lower in resistance than is ultimately desired, in order to permit 

 final adjustment to tolerance. This final adjustment is ordinarily accom- 

 plished by light and uniform abrasion over the entire surface of the resistor, 

 through application of a cotton pad, moistened with an organic solvent, to 

 the surface of the rotating resistor while the resistance is being measured 

 continuously. Measurement has shown that the resistance stability of helixed 

 resistors is slightly increased by this adjustment, probably in part because 

 minute fractures of the film at the groove edges are partially eliminated. 



Fig. 18 — A typical variable pitch helixing machine with cover removed to show pitch- 

 changing mechanism. 



Since the surface irregularities of the core are large relative to the carbon 

 film thickness, abrasion does not remove carbon uniformly from the surface 

 and large increases brought about in this way are often undesirable because 

 of the resulting non-uniformity in film thickness. This non-uniformity re- 

 sults in non-uniform potential gradients over the film surface and thus 

 increases the distributed capacity of the film, which is undesirable in re- 

 sistors to be used at very high frequencies. Non-uniformity in film thickness 

 is also particularly undesirable in resistors designed to dissipate large 

 amounts of power, which may be as great as 30 watts per square inch for 

 hermetically enclosed types or 1000 watts per square inch for Hquid-cooled 

 types. In such resistors, a small high resistance area may result in such 

 pronounced local heating as to fuse the ceramic core locally with resultant 

 progressive failure of a region across the entire conducting path if the power 

 input is maintained. 



