Carbon Film Resistors: Methane Cracked on Ceramic Rods, Helixed and Varnished (Bell Labs 1951)

Ceramic rods tumble in a hot, gas-tight rotating furnace while methane cracks on them and leaves a thin, hard film of carbon. The coated rods get conducting ends, a helix ground through the film to bring them near their value, a light rub to the exact value, and a coat of baking varnish. Result: carbon film resistors adjusted to as close as ±0.5 %. The helix alone can raise a rod's resistance by any factor from about 10 to about 8000. Source: Grisdale, Pfister and van Roosbroeck, Pyrolytic Film Resistors, Bell System Technical Journal 30 (1951). Unknown: the ceramic of the rods, the end caps and leads, and which baking varnish; the paper does not give them. Picture: Wikimedia Commons.