IV. Comparison of simultaneous magnetic disturbances at several observatories

Abstract

In my former on Magnetic Disturbances and Earth Currents, which were read at the ngs of the British Association at Swansea in 1880 and at York in 1881, a iparison was made of the declination and the horizontal force traces given by the self-recording instruments at five European stations, also at one station in India, one in China, and one in Australia. An attempt was made to determine the relative amounts of the simultaneous changes at the several observatories by comparing them with one another by means of the scale values of the instruments employed, which were all of the pattern of the self-recording instruments at the Kew Observatory. It was found on comparison that there were great differences in the scale values of the instruments of the same kind at the different observatories, and in some ca here was great uncertainty as to the scale values, because no determination of had recently been made. Hence great difficulty was found in arriving at the meaning of the records which were taken regularly at the different observatories. The comparison was sufficient to show the great importance of adopting the same scale values for the like instruments at all observatories. In my paper a recomendation was made that for horizontal force records a scale value of ·0005 millimetremilligramme for a difference of scale reading of 1 mm. should be adopted as being the most convenient. The same scale value was recommended by Dr. Wild, of the St. Petersburg Observatory, and for the vertical force magnetometer the same scale value might conveniently be adopted. With this scale value the instruments would be sufficiently sensitive to give for a considerable magnetic disturbance changes which are capable of being measured, but yet would not be so sensitive as to send the spot of light off the photographic paper, even in a violent magnetic storm.