pressboard, well known in the textile and paper processing industries for more than a hundred years, was used for installation purposes in the first electric machines. it was not until the late 1920's, however, that designers became involved with the development of a pressboard type specially designed to meet the requirements of large power transformer construction. the efforts resulted in an insulating material not based on used paper or cotton wastes, as was normally the case, but made with high-grade sulfate cellulose. this special material, called transformerboard, is manufactured from the same material used for the manufacture of insulation for high-voltage cables and capacitor paper. it consists solely of pure cellulose fibers without any binder. it can be completely dried, degassed, and oil impregnated. these and other associated properties are discussed by the authors. attempts were made in the 1930's to manufacture new products such as angle rings, caps, covers, etc from the material which originally had been available only in the form of sheets or rolls. over the years, it has become possible to manufacture moulded insulation parts from transformerboard in dimensions previously regarded as unmanufacturable. angle rings and similar insulation parts molded from transformerboard conveniently combine the electrical advantages of soft paper with the mechanical superiority of resin impregnated kraft paper without exhibiting their disadvantages. in contrast to resin impregnated kraft paper, they do not contain any gas or water inclusions which are unavoidable during bake