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This paper deals with the changes in the condition of glass surfaces when rubbed with flexible materials, chiefly cotton, linen, and silk, of varying purity. The condition is ascertained by two different, but associated, properties :— (a ) The electric charges displayed by the glass surfaces when rubbed on textiles or on one another. (b ) The coefficient of friction found between two of them after being subjected to identical rubbing. We have derived great help in the friction work both as regards theory and technique from the papers on “Boundary Lubrication,” by Sir William Hardy and his colleague (1, 2, 3 4). A solid surface may be modified in two distinct ways :— (1)By Addition of Material .—A solid surface exposed to an atmosphere con­taining water (or other) vapours, condenses and retains these; or if solid or liquid matter be rubbed on the surface, this will in general acquire and retain a covering film. The actual exposed surface is thus a complex of various materials, so that tribo-electric and frictional effects are complicated. We know that the friction of the surface is dependent on the character of the condensed film and on that of the solid underlying the film, according to the expression given by Sir William Hardy and Miss I. Doubleday (2), μ =b —a M, where M is the molecular weight of a condensed pure organic compound,a andb being parameters.

Tribo-electricity and friction