Electrical resistor and method of production

#The electrical art is familiar with the practice of producing electrical resistance elements by applying a film of electrically conductive material over an insulating substrate. Among the electro-conductive film materials employed are various forms of carbon, various metals, and, more recently, oxides. While the electro-conductive films may be deposited on any compatible, electrically insulating substrate, a substantial line of commercial resistors have been developed in recent years wherein a tin oxide film doped with antimony oxide is deposited on a glass substrate. The characteristics of such resistors, their method of production, and the history of their development, are described in U.S. Pat. No. 3,437,974 granted Apr. 8, 1969 to J. Spiegler.

#The resistance value for an electrical film element may be substantially enhanced by selectively removing a portion of the film material to convert the film into an elongated ribbon of material. To this end, it is common practice to deposit the electrically conducting film on a cylindrical substrate, such as a glass rod, and to spiral the element by cutting, or otherwise removing a thin line of film, to form a helical path therein. Originally, resistors were spiraled by a process of mechanical cutting or abrasion such as described for example in U.S. Pat. No. 1,859,112 issued May 17, 1932 to I. Silberstein. This technique is still widely used in the art, but has certain serious disadvantages. For example, the Spiegler patent, mentioned earlier, is addressed specifically to the problem of mechanical damage to the glass substrate from the cutting or abrading operation. The patent proposes a mechanically strong glass substrate which comprises a glass core with a glass sheath on the core, the coefficient of expansion of the sheath being lower than that of the core to strengthen the substrate.

#In addition to the mechanical damage problem described by Spiegler, mechanical cutting or abrasion may produce a jagged edge which limits the electrical precision attainable. Also, there is a distinct physical limit to the width of the path that may be cut in a film, thereby limiting the number of spirals per inch that may be attained and, consequently, the value of electrical resistance attainable in this manner.

#The invention is further described with reference to the accompanying drawings wherein,

#FIG. 1 is a side elevation of a preferred embodiment of a resistor constructed in accordance with the invention, and

#FIG. 2 is a side elevation, diagrammatic view of apparatus for carrying out the invention.

#FIG. 1 illustrates, as a preferred embodiment of the invention, a typical resistor 10 comprising a cylindrical glass rod 12 as a substrate, low resistance metal bands or caps 14 and 16 formed at or over the ends of rod 12 and serving as electrical terminals, and a ribbon 18 of electrically conductive resistance material extending about the surface of glass rod 12 in helical form and defined by a helical path 20.

#Ribbon 18 may, as indicated earlier, be composed of any known material used in the formation of electrically conductive film elements, such as carbon, metals, and metal oxides. However, we prefer to employ a tin oxide film doped with antimony oxide which may be produced and applied in a manner described in detail in U.S. Pat. No. 2,564,706 granted Aug. 21, 1951 to John M. Mochel.

#In a preferred manufacturing procedure, a continuous length of glass cane is drawn from a source of molten glass, and a film of doped tin oxide is applied to the surface of the glass cane at a suitable temperature as the cane cools on the draw. Thus, a suitably proportioned mixture of tin and antimony chlorides may be vaporized in a chamber surrounding the glass cane and the chloride vapors converted to the corresponding oxides, with such oxides being deposited on the glass as described in detail in the Mochel patent mentioned above. The filmed glass cane thus produced may then be cut into suitable lengths for resistor use and caps, or other terminal members, applied over the ends in known manner. Thereafter, it is customary to mechanically scribe or cut a narrow helical path 20 in the film to produce a helical ribbon, such as ribbon 18, of greatly increased resistance value.