NOT REALLY CLEAN

Evening Post, Volume CXVIII, Issue 60, 8 September 1934, Page 5

NOT REALLY CLEAN

VIRTUE OF CREASY PLATE

The friction between scientifically clean surfaces is so enormous that if a clean glass rod is rested on a clean glass plate, the plate can be tilted until it is just short of vertical before the rod will roll down it, says the London "Morning Post." The catch is in the word "clean." What a layman would call a clean object is really covered with a film: of grease so thin that it would take 25,000 of them to make up the thickness of a piece of cigarette paper. It is only when these films are absent that the true effects of friction can be observed. This was explained by Professor E. N. da C. Andrada, Quain Professor, of Physics at the University of London, addressing the British Science Guild at the Eoyal Society of Arts. It would take a trained chemist an hour to wash a cup really free of grease, he said, and then the cup must be kept in a closed vessel, or the surface would be contaminated by matter from the air. This layer of grease normally provides a distinct lubricating effect. This explains why, if a litle tea is spilt in a saucer, the cup will slide about less, because the tea is not such a good lubricant as the grease and interferes with it. • Friction, Professor Andrade described as a feminine force, because it is impossible to say in what direction it will act until you have decided in which direction you want to go, and then it always chooses to act in the opposite one. The scientist is interested in the study of really clean surfaces, because they enable him to study the behaviour of very thin films of liquids, such as oil and grease, and so to investigate the problems of lubrication.