82 



sible to construct identical points, \ve could probably say 

 proportional to their number, if théy are on a certain di- 

 stance from each other. 



As soon as the circuit (point-apparatus — galvanome- 

 ter — earth plates) is closed the galvanometer deviates, sho- 

 wing a current ordinarily from the point-apparatus to the 

 earth but occasionally in opposite direction. The deviation 

 varies very much and the astatique needle-pair of the gal- 

 vanometer is continually in movement. This deviation can 

 have the following causes: l:o from the EMF generated 

 by the contact of the zinkplate with the earth; 2:o from 

 the EMF by the contact of the points with the air and 3:o 

 from the electricity in the air. 



If the both first causes were the principal the devia- 

 tion would be constant or undergo only slovv variations. 

 Now it varies continually and in such a degree that it ch an- 

 ges the sign. The principal part of it must therefore hävt 

 its cause in the electricity of the air. 



Though it is clear that the layer of air, in which the 

 points are situated, exercises the greatest effect by produ- 

 cing of the EMF in the point-apparatus, we can however 

 conclude from the luminous beams, över 100 m. high, which 

 under favourable circumstances were produced above the 

 point-apparatus, that the surrounding airlayers also contri- 

 bute to it or at least that these layers are in an intim ate 

 connexion with each-other. 



The resistance does not only depend on the height 

 över the earth but also on the meteorological circumstances 

 in the air especially on the humidity and the pressure. The 

 changes of the resistance are comparatively slow, depending 

 on the changes in the air. It would however not be just 

 to regard two deviations, separated by an interval of say 

 10 minutes, fit for a calculation of the intensity of the cur- 

 rent because of the probable variation of the resistance. 



In consequence of it we need to introduce the follo- 

 wing method at the observations. 



