The times of recent “sudden commencements” of magnetic storms at Eskdalemuir

Abstract

The publication by Dr. R. Bock of the times of several recent “sudden commencements” of magnetic storms registered at Potsdam, by means of recording arrangements permitting a more accurate determination of occurrence than those in everyday use at most magnetic observatories, suggests that similar information from another observatory may be of interest.Although there is at present at Eskdalemuir (lat. 55° 19′ N.; long., 3° 12′ W.) no means of obtaining continuous open time‐scale registration of the horizontal force or declination, it is possible to give for the last five of the Potsdam cases, and with at least as much accuracy, the times of the “sudden commencements” in the vertical component. These times are derived from records of the rate of change of vertical force, the records being obtained from a sensitive moving‐coil galvanometer connected to the ends of a coil of insulated wire resting in an approximately horizontal plane. The resistance of the circuit is such that the galvanometer is adjusted to critical aperiodicity. This method of registering small changes in vertical force was introduced at Eskdalemuir by Dr. A. Crichton Mitchell some eight years ago, but it is only since the end of January, 1926, that records have been obtained continuously. Particulars of the arrangements and a description of the changes in vertical force during a small “sudden commencement” at 1924, September 4, 5h 45.3m G. M. T., have already been published. For the present purpose it is sufficient to state that on the photographic record the time‐scale is about 10 millimetres to 1 minute, and that a deflection of about 6 millimetres is produced by a rate of change in vertical force of 10−7 c. g. s. per second. Short time‐gaps every minute, and somewhat longer gaps every hour, are made on both the base line and the trace of the galvanometer deflection by an electromagnetic shutter actuated by a contact‐clock which is rated from daily time‐signals from Greenwich. The time of occurrence of a reasonably well‐defined movement on the record can be assigned to well within 0.1 minute.