This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
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.