Carbon Resistance Element: Lampblack in Alkyd and Chinese Wood Oil on Laminated Phenolic

A carbon resistance film for radio sets. Chicago Telephone Supply's Cooper (US 2,120,930, 1938) found that lampblack bound in phenolic resin kept drifting, so he bound it in alkyd resin modified with Chinese wood oil (tung oil). Sprayed on laminated phenolic and baked up to 165 °C, the film becomes insoluble and holds its value in heat and damp. His mix gave about 50,000 ohms in an element of ordinary size, with a film about 0.003 inch thick. The picture: our diagram.