August 25, 1910] 



NATURE 



257 



A fine layer of dust soon becomes firmly imbedded in the 

 metallic surface, so that further sand falling does not come 

 into contact with the metal itself. On the other hand, it 

 is probable that these particles cut through the fibres of 

 the paper, and thus free themselves. 1 need hardly point 

 out that the filter papers used should not be specially dried. 

 Pieces which have been left about in a room for a few 

 hours absorb sufficient moisture to ensure the right degree 

 of conductivity. 



The sign of the charge is always positive, in spite of 

 the fact that a rod of silica rubbed upon the paper electrifies 

 it negatively. In 1843 Faraday had noticed this curious 

 reversal, and briefly refers to it in his experimental re- 

 searches. 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 electrifica- 

 tion is still positive. Further experiments have shown, 

 however, that the sign of the electricity caused by fric- 

 tion against glass or silica depends upon the form of the 

 tubbed surface. For instance, a strip of paper stroked 

 by the smooth side of a tube of either substance becomes 



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 con- 

 stantly 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 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 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 detected 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 com- 

 position 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 men- 

 tion this because it is of interest in connection with the 



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 described, 

 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, say, with 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 

 electrified 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 



NO. 2130, VOL. 84] 



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 instantly to earth, for in common 

 with all powders it readily absorbs moisture into the inter- 

 stices between the grains. When making electrical experi- 

 ments with this material, it is therefore essential to have 

 it well warmed. 



There is still much useful work to be done in studying 

 the electrical conditions in the neighbourhood of wide 

 stretches of warm sand swept by dry wind. Owing to lack 

 of data, it is difficult to form an opinion as to the part 

 which this substance plays in the remarkable electrical 

 phenomena sometimes witnessed during a storm. 



I spoke of allowing sand to run down itself. Here is 

 a cell made by separating two glass plates, 14 inches 

 square, by, strips of wood along the bottom and top edges. 

 The sides are open. Through a hole in the upper distance 

 strip sand pours from a funnel, and builds itself into a 

 beautifully symmetrical conic -section. Presently the base 

 will so far widen that any further increase shoots the 



