Abstract: Mineral insulated conduits are produced by insulating one or several electrical conductors, preferably metallic conductors, with silica staple fibers and sheathing the entire conductor with a metallic jacket. The silica staple fibers are produced by treating dry-spun soda water glass fibers which are free of non-alkaline metal compounds and which have a molar ratio of Na.sub.2 O:SiO.sub.2 of approximately 1:3 to 1:1.9, with aqueous, hydrogen ion containing acid or salt solutions, and then washing, drying, and optionally heat treating. Preferably, a binder is also used. The preferred binders are low in or free of alkalies, such as silica sol, boron trioxide, organosilanes, or the like. The mineral insulated conductors produced are characterized by a low electrical capacitance, low dielectric losses and a high electrical resistivity and are usable in a very wide range of temperatures. Instrument leads, thermocouples and heating conductors may also be produced in this advantageous manner.