Tribo-electricity and friction. IV.—Electricity due to air-blown particles

Electric charges due to mutual impact of dust or other particles arise in various ways: (a ) electric dust- or sand-storms in the Tropics; (b ) electric snow storms in the Antarctic; (c ) electric flashes seen in the ejectamenta from volcanoes; and (d ) electric charges, and possibly sparking, brought about by the raising of organic powders in certain industrial processes. In this group of effects, charges are produced by the impact of a multitude oflike particles. Now, frictional- and impact-charges have ever been regarded as resulting from the shock of twounlike substances. Until recently there was no principle by which charges from "like” substances could be interpreted; but experiments made by the writer throw light on the subject by showing thatidentical solid surfaces can charge one another by friction or impact, and, further, that this property of the surfaces changes as rubbing continues. It was there shown that such organic insulators as ebonite and celluloid yield considerable charges, whereas hard inorganic materials such as quartz, calcite and glass, produce smaller effects. When the “like” solids meet in violent impact, not rubbing, the combined net charge is not nil, as might be expected according to Faraday’s law of equal and opposite frictional charges, but finite, generally negative; so that the air surrounding the surfaces must attain an equal positive charge.