As a consequence of these varying charges, there is a constant
series of currents flowing through the various parts of the earth, which
show themselves in such telegraph wires as may lie in the direction
followed by the currents. Such currents are known as earth currents, and
present phenomena of a highly interesting nature. But, apart from these
electrical manifestations, there is generally a difference of electrical
condition between the various parts of the earth's surface and those
portions of the atmosphere adjacent to or above them. Inasmuch as air is
one of the very best insulators, this difference of condition (or
potential) in any particular region is in most cases incapable of being
neutralized or equilibrated by an electric flow. Consequently the air
remains more or less continually charged. With these points admitted as
facts, the question arises, Whence this electricity? There have been very
many and various opinions expressed as to the cause of terrestrial
electricity, but far the greater portion of such theories lack fundamental
probability, and indicate causes which cannot be regarded as sufficiently
extensive or operative to produce such tremendous effects as are
occasionally witnessed. I take it that we may safely regard the evolution
of electricity as one of the ways in which force exhibits itself, that, in
other words, when work is performed electricity may result.
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