POLAR-LIGHT AS PAR A TUS. 1 59 



polar light or aurora borealis is an electric current flowing from 

 the higher regions of the atmosphere down to the earth. 



A sphere of brass, fixed on a bar of india-rubber or ebonite o - 6 

 metre long, is screwed in the board of the cross-shaped foot. A 

 cylinder of india-rubber, 3 metres long, is fixed to the same board at 

 about 07 metre from the sphere. From the cylinder comes out a 

 branch with a bow, both of india-rubber. On the bow are fixed sixteen 

 Geissler's tubes, wherein the air has a pressure of about 0*5 millimetre. 

 The lower ends of the tubes are pierced by platinum wires, which are 

 directed towards the sphere, whilst at the upper end the platinum 

 wires are, by means of their copper wires, in a metallic union with a 

 button, and also in metallic union with the earth. From underneath 

 the sphere a copper wire, well insulated with india-rubber, leads to the 

 negative pole of a Holtz's electric machine (a machine of Carre' (Paris) 

 was employed with great advantage), of which the positive pole is in 

 metallic connexion with the earth. As soon as the machine is put in 

 movement, the sphere being charged, becomes negative-electric, and 

 at the same time there goes through all the tubes a current of reddish- 

 lilac light, so that they altogether form a shining bow-shaped belt. 

 With an ordinary machine this phenomenon may still be observed 

 when the lower ends of the tubes are at a distance of two metres from 

 the sphere. This proves evidently that the electricity flowing out 

 from (or into) the sphere not only traverses the layer of air that is 

 between, but goes also with such power through the tubes that the 

 gas therein becomes glowing by the heat that the electric current 

 produces, as is well known. In order that the electricity might more 

 easily flow out in the air from the sphere, this latter is furnished with 

 points. These points, as well as the metallic union between the upper 

 end of the tubes and the earth, are of no absolute necessity, for the 

 phenomenon may be produced without them, the distance between 

 the sphere and the tube must, however, then be considerably reduced. 

 The described light-phenomenon produced by the apparatus proves 

 clearly that a current of electricity may go through a layer of air of 

 ordinary pressure 76o mm without producing the light-phenomenon, but 

 if it meets in its way a space of rarefied air of low pressure (from o to 

 3o n;m to 4o mm ) there arises immediately a light-phenomenon, caused by 

 the fact that the current makes the molecules of gas glovr. 





