Cellulose Acetate Treatment for Textile Insulation - Development of the Manufacturing Process

the telephone Bell System a INinsulated wiretheisplant of thelead covered the large quantity of textile employed for wiring switchboards and connecting them to incoming cables. The insulation on this wire varies with its use in the plant, common constructions in the past being tinned copper wire insulated with two servings of silk and one of cotton, or, where the requirements are not so exacting, enameled wire with two servings of cotton. The constant demand for better electrical characteristics in telephone circuits has led the engineers of Bell Telephone Laboratories to seek an improved insulation for this wire as discussed in the contemporary paper, "Cellulose Acetate Treatment for Textile Insulation, Engineering Development," by E. B. Wood and D. R. Brobst. The old design wire was affected by the variation in the dielectric properties of the textile insulation with its moisture content which in turn varied with the surrounding humidity. It was found that the application of a thin film of cellulose acetate to the textile insulation considerably stabilized its properties. The action of the cellulose acetate coating is illustrated in Fig. 1. When the surface of untreated wire is magnified, it is evident that it is a mass of extending fibers and when two conductors lie adjacent, these fibers interlace. Under humid conditions the textile becomes moist and the interlacing fibers afford a path for current leakage. The moist fibers also have a considerably higher dielectric constant than the dry fibers and the electrical capacitance between adjacent wires is increased, therefore, with high humidity.