The January and February numbers of the _Elektrotechnische Zeitschrift_
contain a number of articles on this interesting subject by several
eminent electricians. Professor Foerster, director of the observatory in
Berlin, points out the great importance of the careful study of earth
currents, first observed at Greenwich, and now being investigated by a
committee appointed by the German Government. He further points out,
according to Professor Wykander, of Lund, in Sweden, that a close
connection exists between earth currents, the protuberances of the
sun, and the aurora borealis, and that the nearly regular periodical
reappearance of protuberances in intervals of eleven years coincides
with similar periods of excessive magnetic earth currents and the
appearance of the aurora borealis. The remarkable disturbing influences
on telegraph wires and cables of the aurora borealis observed from the
11th to 14th of August, 1880, have been carefully recorded by Herr Geh.
Postnath Ludwig in Berlin, and a map of Europe compiled, showing the
places affected, with the extent to which telegraph wires and cables
were influenced and disturbed. Although the aurora was but faintly
visible in England and Germany, and in Russia only as far as 35° north,
disturbing influences were reported from all parts of Europe, the
Mediterranean, and Africa, and even Japan and the east coast of Asia.
As far south as Zanzibar, Mozambique, and Natal disturbances were also
noticed. They were in Europe most intense on the morning of August 12,
when they lasted the whole day, and increased again in intensity toward
eight o'clock in the evening, while they suddenly ceased everywhere
almost simultaneously. Scientific and careful observations were only
taken at a few places, but the existence of earth currents in frequently
changing direction and varying intensity, was noticed everywhere. Long
lines of wires were more affected than short ones, and although some
lines--for instance the Berlin-Hamburg in an east-west direction--were
not at all influenced, no general law was noticed according to which
certain directions were freed from the disturbing influence. While, for
instance, the Red Sea cable was not noticeably affected, the land
line to Bombay, forming a continuation of this cable, was materially
disturbed. The Marseilles-Algiers cable, so seriously influenced in
1871, showed no signs at all, but as may be expected, the north of
Europe suffered more than the south, and in Nystad, Finland, the
galvanometer indicated an intensity of current equal to that of 200
Leclanché cells.
Since thunderstorms are generally local, it is only natural that their
effect upon telegraph cables should also be confined to one locality.
Numerous careful observations, carried out over considerable periods of
time, show that the disturbing influences of thunderstorms on telegraph
lines are of less duration and more varying in direction and intensity
than those of the aurora borealis. Long lines suffer less than short
lines; telegraph wires above ground are more easily and more intensely
affected than underground cables. It is, however, possible, that this is
mainly due to the fact that in the districts where strict records were
kept, in the German Empire, most of the long lines are underground
cables, while most of the short local lines are overground wires. The
results of the disturbances varied; in Hughes's apparatus the armatures
were thrown off, lines in operation indicated wrong signs, dots became
dashes, and the spaces were either multiplied in size or number,
according to the direction of the earth currents induced by the
thunderstorms. Since these observations extended over nearly 2,000
cases, some conclusions might fairly be drawn from them. For the purpose
of a more complete knowledge on this subject, Dr. Wykander recommends a
series of regular observations on earth currents to be carried out at
different stations, well distributed over the whole surface of the
globe, these observations to be made between six and eight A.M., and at
the same time in the evening. Special arrangements to be made at various
stations to record exceptionally intense disturbances during the
phenomena of the aurora borealis, notice to be taken of time, direction,
intensity, and all further particulars. Since this question appears to
bear a considerable amount of influence on underground cables, it is one
that deserves serious attention before earth cables are more generally
introduced; there can, however, be little doubt that they are not nearly
so much exposed as overhead wires to disturbing influences of other
kinds, such as snow, rain, wind, etc., while they certainly do
suffer, though perhaps in a less degree, by electrical
disturbances.--_Engineering_.
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