Assumption of a Solar Electric Potential 171 
earth's polar regions may be that the polar air is more strongly 
electric, as it is continually displaced by the more strongly 
electrified air brought by the equatorial current in the upper 
regions of the atmosphere, and must therefore receive in its 
entire mass the electricity of the highest strata of air of the 
lower latitudes. Earth-currents, standing in intimate con- 
nexion with the northern and southern lights, are then to be 
considered a necessary consequence of the compensation of the 
variations of intensity of the electricity of the earth and the 
sun taking place preeminently in the polar regions. These 
compensation-currents must on their part affect the magnetic 
needle by their electrodynamic action. 
Here, however, the question intrudes, is not the earth's 
magnetism itself to be considered an electrodynamic action of 
the electrical charge of the earth ? According to the beautiful 
investigation instituted in Helmholtz's laboratory, under his 
direction, by Mr. Rowland*, it is to be regarded as demon- 
strated that stationary electricity mechanically moved exerts 
electrodynamic actions in a similar manner as an electric 
current. Accordingly the earth must, if its surface is charged 
with electricity of great density, exhibit, in consequence 
of its rotation, magnetic phenomena, in like manner as if 
electric currents encircled it, carrying round it in each lati- 
tude, during the time of a rotation, just so much electricity 
as the static electricity present on the respective surface-rings 
amounts to. What density of electricity on the earth's surface 
would be required, in order to produce the magnetism of the 
earth by its rotation, will not be difficult to practised mathe- 
maticians to calculate. As the magnetic moment of a circular 
current is proportional to the surface flowed round, we may 
foresee that, considering the dimensions of the earth, that 
density will not prove to be inadmissibly great. Further, with 
the colossal dimensions of the sun, whose surface contains 
11,483 times the surface of the earth, while the distance of 
the sun amounts to only 22,934 earth's semidiameters, the 
density of the sun's electricity need be only about twice that 
of the earth in order to call forth the latter by electric dis- 
tribution. Were the whole of the earth's surface equally 
charged with electricity, the magr etic pole would necessarily 
coincide with the pole of rotation of the earth. Smce it is 
not so, and as, on the whole, great irregularities take place 
in the distribution of the earth's magnetism over its surface, 
the distribution of the static electricity on that surface must 
be irregular. This also appears probable when it is. re- 
membered that about one third of the earth's surface consists of 
* Monatsberichte der Berliner Akademie der Wissenschaften. 
