310 METEOROLOGY AND ALLIED SUBJECTS. 



Nahrwold has investigated the conditions under which the atmos- 

 phere can assume a charge of electricity. He used an apparatus con- 

 sisting essentially of the following : A metal cylinder in which the air 

 to be investigated can be entirely closed and removed from the influence 

 of exterior electrified bodies. Through an aperture in the upper cover 

 of the cylinder a filter filled with quicksilver is inserted, which is con- 

 nected by a wire with the quadrant electrometer. If the air in the 

 cylinder is electrified, and we allow the mercury to flow, the needle of 

 the electrometer shows a deviation. The mercury acts like Thomson's 

 water-dropping apparatus. The cover of the cylinder has one other 

 opening through which can be successively introduced two different 

 arrangements for electrifying the included air : first, a needle with a 

 fine point that can be pushed in and out of a platinum tube; second, a 

 thin, short platinum wire soldered between two copper wires: in the 

 experiments these copper wires are connected with the battery so that 

 the transmitted current brings the platinum wire to incandescence. 



If the cylinder is filled at the window with fresh air, the electrometer 

 put in position, the needle shoved forward, and we allow the scarcely 

 visible spark from a feebly-charged Ley den jar to jump to the wire con- 

 nected outside the cylinder with the needle, and then set the dropping 

 apparatus at work, there is observed a sensible deviation of the elec- 

 trometer. If the experiment is repeated with the intention of attaining 

 a stronger charge, we, on the contrary, observe now a feebler deviation. 

 This diminishes continually so long as we retain the same mass of air 

 in the apparatus, and not only when it is electrified but also when it is 

 allowed to stand quietly. We must seek the cause of this phenomenon 

 in the behavior of the particles of dust. So long as the dust is circu- 

 lating within the metal cylinder it rapidly carries away the electricity 

 collected at the point of the needle; but if the dust has settled (and this 

 is very much retarded by the electrification), then is the air freed of dust 

 not in the condition under the given circumstances to take uj) a charge. 

 If by means of a feather we stir up the dust from the sides of the cyl- 

 inder, then the first subsequent experiment shows again a large devia- 

 tion; if, on the other hand, we hold every particle of dust that touches 

 the sides of the cylinder firmly there, to which end Nahrwold covers the 

 interior of the cylinder with glycerine, then the further experiments 

 show that the dropping apparatus takes almost no further charge. 



Instead of the needle point, the incandescent platinum wire is now 

 employed to give a charge to the included air. The battery that serves to 

 excite the electric current requires no further consideration in this case; 

 it, together with the conducting wire, is isolated, and the platinum wire 

 within the cylinder can be charged from any source of static electricity. 

 The air now takes up the electricity even if it is entirely free from dust 

 The air heated by contact with the glowing platinum wire becomes a 

 good conductor of electricity ; it flows away cooling down, but still re- 

 taining its charge, while other portions of the air within the cylinder 

 become heated and charged with electricity by the platinum wire. 



