GB824233A - Film resistor element and method of fabricating same - Google Patents

Abstract

An element for a variable resistor comprises a resistive film formed directly on and bonded to the granular surface of a dielectric; said film being of uniform thickness of a lower order of magnitude than the mean grain size of the surface. In the embodiment described a disc of unglazed ceramic such as steatite is ground with 120 microns silicon carbide grit followed by 22.5 microns Al2O3 grit to provide a surface having a grain size of the order of 20 microns. The smaller area between the terminals is then coated with a specified glaze, before a part ring of noble metal or nickel-chromium-aluminium-copper alloy is vacuum evaporated on the unglazed part of the disc. The thickness may be between a few molecules and a few microns. The disc is next rotated on a spindle at 80 r.p.m. and sprayed during one revolution with a selfhardening drying oil, silicone varnish or preferably a 5% solution of sodium silicate, being wiped smooth by a graphite brush diametrically opposite the nozzle, to expose the metal film at many minute areas. Dry air at 400 DEG F. is then blown on to the surface for the next 15 revs. with the brush in place and for 30 revs. with the brush removed. The coating may now be repeated. Specification 824,234 is referred to.