The measurement of variations in the Earth's magnetic field

There does not seem to be at present any systematic method in general use for measuring simultaneously very rapid changes in all three components of the Earth's magnetic field. The suspended magnetic needle gives only an integrated value of the couple acting upon it, for since its moment of inertia is always considerable, its period is always great in comparison with such rapid changes. There is, therefore, no evidence from this source for or against rapid changes. A method is described in Terrestrial Magnetism (vol. 12, pp. 1–14, 1907), by Ebert, whereby rapid changes in the vertical component Z can and have been measured; the first part (I) of the present paper is to show that by a slight extension of Ebert's method we can determine with accuracy the motion of the magnetic‐force vector during normal days, and, what is more important, determine the motion of the vector during a magnetic storm.