. Nature . fied. As the charge increases upon this, apoint is reached when some of the impinging sand particlesbecome deviated by repulsion, so as to completely miss the potential falls below the critical value, a reverseaction talces place, and the plate rapidly charges up. Turning for a moment to the question of the electrifica-tion produced in sand by the friction between the grains,experiments upon this point may be conveniently made bycatching the particles, which roll down the surface of asand cone, upon a small wet insulated table. Anyelectrification of the latter may then be dete
. Nature . fied. As the charge increases upon this, apoint is reached when some of the impinging sand particlesbecome deviated by repulsion, so as to completely miss the potential falls below the critical value, a reverseaction talces place, and the plate rapidly charges up. Turning for a moment to the question of the electrifica-tion produced in sand by the friction between the grains,experiments upon this point may be conveniently made bycatching the particles, which roll down the surface of asand cone, upon a small wet insulated table. Anyelectrification of the latter may then be detected in theusual manner. If the grains are all of the same nature,we should not expect to find other than slight irregularcharges. The friction between particles differing in com-position would give more definite results. Thus white sandracing over iron sand might be expected to show a charge ;but experiment gave only a feeble electrification. I men-tion this because it is of interest in connection with the. negatively electrified, whereas if the sharp edges of theend graze the paper, the sign of the electrification of thelatter is positive. Now sand consists of sharply angularparticles of silica, and even the comparatively large piecesobtained by crushing the tube, as previously described,have razor-like jagged edges. We should therefore expect,from the result of the experiments just mentioned, thatwhen either sand grains or even large silica chips fallupon paper they will electrify it positively—and this is whatactually occurs. Why an edge of glass should give anopposite charge to that produced by a flat surface whenrubbed, say, with paper, is a question of great interestand difficulty. But that this is ■ the explanation of thestrange electrical behaviour of practically all powdersappears certain. The sand grains themselves become, of course, negativelyelectrified after striking the paper, so that this is often aconvenient method of obtaining a high potential of eithersig
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