I will try to give a short report of some experiments I have made during
the last year in regard to atmospheric electricity. It was formerly
uncertain whether the electrostatic potential would increase by rising
from the surface of the earth to more elevated region of the atmosphere
or not, and also whether the potential in a normal--that is,
cloudless--state of the atmosphere was always positive or sometimes
negative. Sir William Thomson found by exact methods of measuring that
the increase of the potential with elevation is very important, and
values about 100 volts per meter. That fact is proved by many other
observers, especially lately by Mr. F. Exner, at Vienna, who found an
increase of 60 to 600 volts per meter. The observations were made by
means of an electrometer. In respect of many inconveniences which are
connected with the use of an electrometer, I have tried the measurements
with a very sensitive galvanometer. In this case it is necessary to
apply a separating air exhaust apparatus, for example flame, or a system
of points at the upper end of the conductor, which is elevated in the
atmosphere. In order to get a constant apparatus, I have used 400 of the
finest needles inserted in a metallic ribbon. This system I have raised
in the air by means of a captive balloon, or by a kite, which was
attached to a conductor of twine or to a twisted line of the finest
steel wire. In this way I have attained a height of 100 to 300 meters.
When the lower end of the kite line was communicating with the
galvanometer whose other terminal was in contact with the earth, a
current passed through the galvanometer. For determining the strength of
this current I proposed to called a micro-ampere the 10^{-9} part of an
ampere. At the height of about 100 meters in the average the current
begins to be regular, and increases at the height of 300 meters to 4,000
or 5,000 of these units. The increase is very regular, and seems to be a
linear function of the height. I have, nevertheless, found the smallest
quantities of dust contained in the atmosphere or the lightest veil of
cirrus disturbed the measurement very materially, and generally made the
potential lower. In negative experiments of this nature I have made at
Breslau, at the Sohneekoppe, and at the "Reisengebirge," especially at
the last station, an increase of potential was observed, not only by
reason of the perpendicular height, but also by reaching such regions of
the atmosphere as were situated horizontally to about 200 meters from
the utmost steep of the same mountain, Sohneekoppe. Therefore it must,
according to Mr. Exner, be assumed that the surface of the air presents
a surface of equal potential, and that the falling surfaces of high
potential were stretched parallel over the plane contours of the air,
and more thinly or narrow lying over all the elevated points, as, for
example, mountains, church towers, etc. On the basis of these facts I
think it easy to explain the electricity of thunder storm clouds, in
fact every cloud, or every part of a cloud, may be considered as a
leading conductor, such clouds as have for the most part perpendicular
height. After being induced the change results by supposing the
conduction of electricity either from the upper or from the lower side,
according to greater or smaller speed of the air in the height. In the
first case the clouds will be charged positive, in the other negative. I
am inclined, therefore, to state that the electricity of thunder storm
clouds must be considered as a special but disturbed case of the normal
electric state of the atmosphere, and that all attempts to explain
thunder storm electricity must be based on the study of the normal
electric state of the atmosphere.
* * * * *