Triboelectricity and friction. VII.—Quantitative results for metals and other solid elements, with silica

Abstract
For research in triboelectricity (i. e., frictional electricity), metals and silica are pre-eminently suitable; they are hard, chemically simple, and their surfaces can easily be cleansed. Systematic work on these materials is described in a previous paper of this series. We have gone over the ground again using improved apparatus and methods; and the results, which in the earlier paper were little better than qualitative, may now claim quantitative rank. Comparison of the two sets of results shows that they are in substantial qualitative agreement. The process throughout this series of papers is to rub one solid on the other in a machine which maintains constant pressure and constant length of rub; in the present case a metal or other rod is rubbed on a silica tube. The silica is surrounded in part by a metal inductor connected by wire to an electrometer, so that the charge produced by friction on the silica induces a charge on the inductor and electrometer. A reading is taken on the electrometer; and by calibrating the system with known voltages, the electrometer readings are known in volts. Since the capacity of the charged system remains constant, the charges generated vary as the readings in volts. The former method of rubbing the surfaces is shown in fig. 1 (a) where the upper rod A in its stroke bears with one spot on a succession of spots of the lower rod B. Thus the single spot in the upper rod rubbed throughout the stroke becomes hotter and more strained than any spot on the lower rod. Such differential treatment of the two surfaces with its complicating effect on the physical action of the friction, we now avoid, as follows: place the lower rod at 45° to the line of run of the upper one, fig. 1 (b), so that when A moves forward, a succession of points on it come in contact with a succession of points in B. In this way the treatment of the two surfaces is equalised.

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