From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Insulating materials, focusing on the disadvantages of wood and the properties of Vulcanite.
| Identifier | ExFiles\Box 24\2\ Scan095 | |
| Date | 25th January 1921 guessed | |
| - 8 - only permissible to use it under oil. It is inflammable and affected by heat and the conditions of the surrounding air. Contraction and distortion are met with even in planks that have been dried for many decades, These drawbacks cannot be removed by impregnation. Extensive tests have been made in America on networks up to 12,000 volts with insulations made of maple, eucalyptus, oak and acacia wood. They were impregnated with asphalt, oil or paraffin. It may be supposed that these tough resinous woods are nearly unbreakable, that they do not crack or fracture and are easy to transport. Nevertheless the insulation deteriorates considerably after a few years. Insulators act like condensers. First, invisible silent discharges commence, then brush discharges grow until sparks pass. Ozone and nitrous oxide, acting together with the moisture of the air attack the surface of the wood. The impregnation may perhaps slightly retard the destruction, but cannot prevent it. At last the vegetable fibres begin to disintegrate and the surface shows a kind of carbonisation and destruction is complete as far as the electrical and mechanical properties of the insulator are concerned. On this account the use of wood for insulators has been discontinued in America. Vulcanite is an insulating material of high quality. It is made of rubber, the hydro-carbon compound, which is extracted with the sap of certain trees that grow in countries with hot climates, that is to say temperatures of 30 to 40 degrees and high humidity of the earth. Marsh lands of this type are found in Brazil, Central America, Siam, Java and Central Africa. The tree stems are generally very high and thick and are cut either in horizontal or vertical slits through which the sap runs out. It is caught in vessels and the solid rubber separated and dried. The constitution is very variable according to the place of origin and the presence of a series of impurities. The best quality is the para kneaded together by machines under the influence of slight warmth so that they unite into large thick lumps. The material is quite unaffected by water, alkalies and acids, but has the great drawback of changing its state greatly with variations of temperature. Rubber becomes hard and brittle under the influence of cold, and sticky under the influence of sunlight. It swells in hot water, becoming lighter in color during the process. The melting point is at about 120 degrees, at which temperature it changes into a tar-like mass and at 200 degrees it decomposes with the evolution of combustible vapours. Its commercial use commenced in the year 1839, when Goodyear found that a horn-like material could be made by treating rubber with elementary sulphur at a temperature of about 120 to 140 degrees. This process is called vulcanising. It is important for this process that the rubber which has been previously treated with quickline | ||