520 Intelligence and Miscellaneous Articles. 



The strongest currents were shown by zinc, and the feeblest by 

 copper. 



The currents are, however, difficult to observe, doubtless owing 

 to the facility with which they unite across the plates themselves ; 

 more powerful effects may be obtained by cutting a knee-piece iu 

 the metal plate, in one branch of which the long grain is across 

 and in the other in the direction of the length, and then joining 

 the two ends of the knee-piece to the external circuit. — Journal cle 

 Physique, April 1886. 



ON THE ORIGIN AND THE LAWS OF ATMOSPHERIC ELECTRICITY. 

 BY PROF. FRANZ EXNER. 



After a critical historical discussion of previous theories, the 

 author communicates the results of his experiments on this subject, 

 which have extended over several years. They refer chiefly to an 

 examination of the electrical field of the earth under normal atmo- 

 spheric conditions. It has appeared that the equipotential surfaces 

 are continually as if the Earth had a negative charge. Over a 

 plane the fall of potential is always linear, and its absolute value 

 is constant if the condition of the atmosphere does not change. 

 Such changes are due to the greater or less quantity of aqueous 

 vapour in the air, for this leaves the Earth charged with negative 

 electricity. 



The greatest fall of potential amounts to 600 volts per metre in 

 the complete absence of aqueous vapour, and sinks to below 100 

 volts in the height of summer. These numbers hold, however, only 

 for continuous fine weather. A measurement of the fall of poten- 

 tial in summer at great heights, by means of air-balloons, has 

 proved that this increases considerably with the height; that is, 

 that aqueous vapour in air is really negative. 



All the phenomena of negative electricity are completely explained 

 on the assumption already made by Peltier that the Earth contains 

 a negative charge. Starting from Eranklin's theory, this charge 

 appears as a necessary consequence of the gradual formation of the 

 Earth, and would correspond to an excess of electricity above the 

 normal amount. Bodies which are in the condition last named 

 would thus appear negatively electrified. Erom the magnitude of 

 the fall of potential at the surface of the Earth, the absolute poten- 

 tial of the Earth may be determined. This is found to be = — -4-10 9 

 volt ; that is to say, a point in space which is infinitely distant 

 from all electrical masses has a potential which is 4*10 9 volts higher 

 than that of the Earth. The repulsive force which is exerted by 

 the charge of the Earth on a square centimetre of surface is equal 

 to 16*10 -9 grams, and is thus extremely smalk — Sitzungsberichte der 

 Jcaiserliclien Akademie der Wissenschaften in Wien, Eebruary 1886. 



