1. Potsdamer erdmagnetische Kennziffern;BABTELS;Ztschr. f. Geophysik,1938
2. This paper introduces the K-index for Potsdam-Niemegk and a series of fourteen communications giving tables for K. A few of them contain summaries and discussions: Ztschr. f. Geophysik 15, 214–221 (1939); 16, 185–194 (1940); 17, 317–327 (1941/42), dealing with the international adoption of the K-index.
3. The three-hour-range index measuring geomagnetic activity;BABTELS;Terr. Magn.,1939
4. This paper followed the tentative introduction of the K-index at the Washington 1939-Meeting of the International Association of Terrestrial Magnetism and Electricity. Its present value consists mainly in the collection of typical magnetograms from seven widely distributed observatories (see especially p. 419 and pp. 422–423 with plots of non-K variations, and the magnetograms with solar-flare effects). Some of the parameters introduced in that paper (such as Km, B) have later been improved upon: Km has been replaced by Kp, and the daily index B has been replaced by the planetary character-figure Cp and the equivalent amplitude Ap.
5. Main features of daily magnetic variations at Sitka, Cheltenham, Tucson, San Juan, Honolulu, Huancayo, and Watheroo;BABTELS;Terr. Magn.,1939