Abstract
Royal Society, February 3.—The following papers were read:—“Note on an Extension of the Comparison of Magnetic Disturbances with Magnetic Effects inferred from observed Terrestrial Galvanic Currents—and Discussion of the Magnetic Effects inferred from Galvanic Currents on days of tranquil magnetism.” By George Biddell Airy, Astronomer Royal. (Received December 22, 1869.) The author, referring to his paper in the Philosophical Transactions for 1868, stated that he had examined the whole of the galvanic currents recorded during the establishment of the Croydon and Dartford wires (from 1865 April 1, to 1867 October 24). The days of observation were divided into three groups: No. 1 comprising days of considerable magnetic disturbance; No. 2, days of moderate disturbance, of which no further use was made and No. 3, days of tranquil magnetism. The points most worthy of notice are, that the general agreement of the strong irregularities, galvanic and magnetic, is very close; that the galvanic irregularities usually precede the magnetic, in time and that the northerly magnetic force appears to be increased. The author remarks that no records appeared open to doubt as regards instrumental error, except those of western declination; and to remove this he had compared the Greenwich curves with the Kew curves and had found them absolutely identical. In the discussion of the galvanic current-curves, on clays of tranquil magnetism, for independent examination of the galvanic laws, the author explained the method of measuring the ordinates and connecting the measures into expressions for magnetic action, at every hour, grouping the measures, at the same nominal hour, by months and taking their monthly means for each hour. As these exhibited sensible discordance, they were smoothed by taking the means of adjacent numbers, taking the means of the adjacent numbers of the new series and so on, repeating the operation six times. The author explained the theory of this process and the way in which it tends to degrade the periodical terms of higher orders. He then explained an easy method of resolving the numbers so smoothed, into periodical terms recurring once or twice, or thrice in the day, &c. and applies the method to the numbers for every month. When these quantities (which from month to month are perfectly independent) are brought together in tables, they present such an agreement, with gradual change accompanying the change of seasons, as to leave no doubt of their representing a real law of the diurnal changes of the galvanic currents. They also show the existence of a constant turn towards the north (explaining the apparent increase of force to the north observed in the results for days of great disturbance), and a still larger force towards the west (also well marked on the days of great disturbance). No light is obtained as to the origin of these turns; but they appear to be probably pure galvanic accidents, depending on the nature of the earth-connections. The author then exhibited, in curves, the diurnal inequalities of magnetism which the galvanic currents must produce. The form generally consists of two parallel lobes, making with the magnetic meridian an angle of nearly 60° from the north towards the west. The greatest east-and-west difference of ordinates, in the month of April, is 0.00044 of total horizontal magnetic force; it corresponds in the hours to which those ordinates relate, nearly with the ordinary diurnal inequality. But it is much smaller than the ordinary diurnal inequality and the daily law of the galvano-magnetic inequality differs greatly from that of diurnal inequality. For the greater part, therefore, of diurnal inequality the cause is yet to be found.
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Societies and Academies . Nature 1, 390–394 (1870). https://doi.org/10.1038/001390a0
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DOI: https://doi.org/10.1038/001390a0