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).
The properties and uses of different electrical insulating materials such as slate, marble, mica and wood.
Identifier | ExFiles\Box 24\2\ Scan094 | |
Date | 25th January 1921 guessed | |
- 7 - Slate has only a moderate insulating strength and is liable to split up in layers, but is more easily worked than marble. There are of course large differences of qualities in various samples of both these materials. Italian marble is more crystalline than German and its working therefore much more difficult. It is superior to the latter in electrical qualities. Furthermore white marble is a better electrical material than the colored varieties. The marble, which is a chemical compound of chalk and carbonic acid, is easily attacked by weak acids, water vapour and smoke. At a temperature of 30 degrees it has already a considerable conductivity. Experience shows that natural products, whose reliability is variable, have been always discarded by manufacturers and replaced by manufactured articles that can be depended upon and that often prove superior in their behaviour to the natural material. On this account the use of marble and slate as insulators is decreasing rapidly. One of the most important insulating materials is mica. It consists of silicate of alumina with alkaline silicates and magnesia crystallised monoclinic-ally in six sided prisms with angles of 120 and has a hardness of 2.7 to 3. Particularly pure mica is found in India. Natural mica is one of the best dielectrics that we possess. Plates of 1/10" thickness stand 40,000 volts and more. As natural mica is very costly when in large pieces, such pieces are constructed of small portions of mica by using the property of mica that it can be split up into layers. This is very easy in parallel planes, so that very thin, elastic and pliable pieces of mica are obtained which are only several hundredths of a m.m. thick. Applications of various process of wrapping upon cylinders or compressing produce various solid insulating materials by the use of small pieces of mica held together by the smallest possible quantity of shellac and other highly insulating materials. These products are known as micanite, megotalk, megomite, and micarta, etc. Their use in all possible forms as cylinders, slot-insulation, washers, rings, coils, tubes and plates is well known. These compressed products are slightly less insulating than natural mica, but approach it very nearly in good manufactures. Their capacity for absorbing moisture and change in insulating quality at the normal temperature reached in machined is comparatively small. On this account mica products are highly valued in high voltage installations and in the construction of electrical apparatus. Wood is a natural insulator. It was greatly used in the times when electrical engineers were only concerned with feeble currents. But since the advent of installations with higher voltages its use has been discontinued and it is contd. | ||