89 



intensity; a new important proof of the electrie nature of 

 auroras. 



The forces which produce the el. air-current are, small 

 if taken absolutely, especially if \ve compare them with the 

 forces given by the electrometer. The former shows ordi- 

 narily tenth of volt when the latter attains ten to hundreds 

 of the same, but as surprising as it may be, it is a fact 

 resulting from direct observations. 



Here the objection can be made: that the effects of 

 the artificial point-apparatus can not be compared with 

 what is going on in the nature itself, if the apparatus was 

 absent. 



This objection will nearest hit one side of the whole. 

 namely the quantity of electricity which flows down, but 

 not the EMF or pressure by which the flowing is accom- 

 plished. The point-apparatus with ils wire to the earth can 

 be compared, as said above, with a tube between two re- 

 servoirs of water, the one situated a little higher than the 

 other. The quantity of watar, running from the upper to 

 the lower reservoir in a minute, depends on the differenee 

 of hight of the two reservoirs or the pressure on the one 

 side and on the section of the tube on the other. The for- 

 mer or the pressure will be unaltered, how the size of the 

 section of the tube may be changed and in the same way 

 behaves the EMF of the air-current. 



Under ordinary circumstances the el. air-current will 

 arrive principally by the unevenesses on the surface of the 

 earth; the more numerous and sharp they are, the more 

 electricity will flow trough them. The point-apparatus will 

 increase the quantity verv much, but the EMF will be the 

 same in both cases. The observations with the point-appa- 

 ratus give a relative measure of what is going on in the 

 nature on a certain surface around the apparatus. 



7:o. As it is said above, the fact, discovered by Wu- 

 kander, that the magnetic perturbations change their signs 

 on apposite sides of the auroral belt, will find a simple ex- 

 planation in the currents from the atmosphere, appearing 



