1010] on Electrical and other Properties of Sand. 745 



In 1843 Faraday had noticed this curious reversal, and briefly refers 

 to it in his Experimental Researches. Even if the actual silica 

 rod be broken up into pieces, say as large as an orange pip, and 

 allowed to fall upon the paper held obliquely, the sign of the electri- 

 fication is still positive. Further experiments have shown, however, 

 that the sign of the electricity caused by friction against glass or silica 

 depends upon the form of the rubbed surface. For instance, a strip 

 of paper stroked by the smooth side of a tube of either substance 

 becomes negatively electrified, whereas if the sharp edges of the 

 end graze the paper the sign of the electrification of the latter is 

 positive. Now sand consists of sharply angular particles of silica, 

 and even the comparatively large pieces obtained by crushing the 

 tube, as previously descrilaed, have razor-like jagged edges. We 

 should therefore expect, from the result of the experiments just 

 mentioned, that when either sand grains or even large silica chips 

 fall upon paper they will electrify it positively — and this is what 

 actually occurs. Why an edge of glass should give an opposite 

 charge to that produced by a flat surface, when rubbed sa}- witli 

 paper, is a question of great interest and difficulty. But that this is 

 the explanation of the strange electrical behaviour of practically all 

 powders appears certain. 



The sand grains themselves become of course negatively elec- 

 trified after striking the paper, so that this is often a convenient 

 method of obtaining a high potential of either sign. Further, a 

 stream of sand falling upon a metal plate will give a comparatively 

 low potential, say 600 volts for an indefinite period, in spite of 

 pitting, and a tolerably steady value may be obtained by catching 

 the grains upon a second disc (previously dulled by a sand blast) 

 connected with the apparatus required to be constantly electrified. 

 As the charge increases upon this, a point is reached when some of 

 the impinging sand particles become deviated by repulsion, so as to 

 completely miss it. If the potential falls below the critical value a 

 reverse action takes place, and the plate rapidly charges up. 



Turning for a moment to the question of the electrification 

 produced in sand by the friction between the grains, experiments 

 upon this point may be conveniently made by catching the particles, 

 which roll down the surface of a sand cone, upon a small wet 

 insulated table. Any electrification of the latter may then be de- 

 tected in the usual manner. If the grains are all of the same nature 

 we should not expect to find other than slight irregular charges. 

 The friction between particles differing in composition would give 

 more definite results. Thus white sand racing over iron sand might 

 be expected to show a charge. But experiment gave only a feeble 

 electrification, I mention this because it is of interest in connection 

 with the atmospheric electrical phenomena which often accompany 

 sand storms in hot climates. Even if the wind electrified the surface 

 of the sand over which it blows, the charge would probably leak 



