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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



roughness required for good adherence generally increases with film thick- 

 ness, the fihns invariably being under lateral compression at normal opera t- 



BEFORE HELIXING 



AFTER HELIXING 



Fig. 17 — Mechanical imperfections in ceramic rods, carbon coated. 



ing temperatures. It has been found that crystalline substrates such as 

 porcelain, zirconium silicate and alumina, which may be made nonporous, 

 provide excellent adherence. Vitreous surfaces such as those of overfluxed 



