From 1931 until the second year of World War II, he was also a research associate at the Carnegie Institution of Washington.
He collaborated with Sydney Chapman to publish the two-volume work Geomagnetism, a definitive reference on geophysics.
[1] In 1933, Bartels signed the Vow of allegiance of the Professors of the German Universities and High-Schools to Adolf Hitler and the National Socialistic State.
When, in 1958 International Council for Science, created the Committee on Space Research (COSPAR), Bartels became chairman of the West-German branch.
[2] Among his contributions was the development of the Kp-index, and he suggested the existence of "M-regions" on the Sun that resulted in geomagnetic activity.