ATMOSPHERIC ELECTRICITY. 95 



even though the tube L is lengthened by an addition of another piece 

 3 feet or 4 feet long. 



Several phenomena, one of which had been known for a long time, 

 can be exidained by the fact that the electric discharge changes the 

 condition of the gas into a state similar to that of gases rising from 

 flames. It is mentioned, for instance, by Faraday that electric sparks 

 are liable to succeed each other along the same path, and it is known that 

 the same holds for lightning flashes, facts which themselves point to a 

 higher conductivity of air along the path of the previous discharge. A 

 curious instance of a similar effect is afforded by lightning conductors, 

 which are sometimes put up to protect overhead leads used for con- 

 veying a high tension current. Owing to the obvious impossibility 

 of connecting the leads directly to earth, a small air gap is inter- 

 posed, the idea being that the air gap will act as an insulator for the 

 current the leads are intended to carry, but that if during a thunder- 

 storm the potential rises sufficiently high to be dangerous, equalization 

 may take place through the air gap to earth by means of a small spark. 

 So far the air gap answers its purpose, but as soon as a spark passes 

 through the gap it destroys the insulating power of the air, and the 

 main current consequently takes a short cut through the gap. At 

 Pontresina, in the Engadine, lightning conductors put up in this way 

 are so sensitive that a flash of lightning several miles away causes a 

 small spark by induction, and instantaneously puts out every electric 

 lamp in the town. 



If we accept the view that an electric discharge destroys the insulat- 

 ing power of the gas, it follows that the outer regions of the atmosphere 

 must conduct, for we have ample reason to suppose that electric cur- 

 rents are passing continuously through those regions. The aurora 

 borealis in the arctic regions is, according to Nordenskiold's observa- 

 tions, a permanent phenomenon, and the diurnal changes of terrestrial 

 magnetism show that in our latitudes electric currents traverse the air 

 above us. However small a conductivity we may assign to the atmos- 

 phere, the earth could not remain electrified inside such a shell of par- 

 tially conducting gases. Lord Kelvin drew the same conclusion in the 

 Eoyal Institution lecture, on the assumption that gases at much reduced 

 pressures cease to insulate. We may leave it an open question whether 

 the normal electric stress could in itself cause a discharge in the outer 

 regions; but we can not deny that under existing conditions these 

 regions do not insulate, and Lord Kelvin's argument still holds good. 



But the question of the ending of the lines of force — in other words, 

 the location of the positive charge corresponding to the negative elec- 

 trification of the surface of the earth — can only be solved by balloon 

 or kite experiment, and we may briefly mention the more important 

 results which have so far been obtained. 



Observations made up to heights of about 1,000 feet seem to indicate 

 a strengthening of the electric field, i. e., the fall of potential per meter 



