l I N T R O D U C T I O N T O V O L U M E 1 5 relation between gravitation and electromagnetism, as his repeated complaints for failing to formulate such a theory testify (see his letters to Auguste Piccard, Ehren- fest, and Kaluza, Vol. 14, Docs. 379, 384, 444). An episode in the present volume allows a glimpse of his effort to forge such a connection. It was at the meeting of the GDNÄ in Düsseldorf, 19–25 September 1926, that the young physicist Teodor Schlomka approached Einstein about a plan to test whether the geomagnetic field is due to the rotation of the Earth by measuring the second derivatives of the geomagnetic potential, and Einstein talked about his idea of bulk matter behaving like an electrically charged mass having a “ghost charge” and producing a magnetic field when in rotation (see Doc. 442, and Introduction to Vol. 14, sec. V).[14] Schlomka subsequently initiated a correspondence with Einstein on his planned experiment. He offered to verify Einstein’s idea in an airplane and to find out whether and how the direction of the observed geomagnetic field in motion with respect to the Earth deviates from the direction found on the ground. He also en- listed various theories of Ottaviano Mossotti, Friedrich Zöllner, Lorentz, Arthur Schuster, and William Swann intended to explain the Earth’s magnetic field by claiming a tiny difference between the absolute charges of the electron and the pro- ton, just sufficient to get a surplus attraction and electric charge, which might ac- count for gravitation and geomagnetism, respectively. Einstein, too, believed that rotation of bulk matter alone could not be the source of the geomagnetic field, since otherwise translational motion should also produce a similar effect, which, howev- er, had not been observed by ships crossing the Atlantic from Europe to America (Doc. 475). He thought that Maxwell’s vacuum equations ought to be modified in order to obtain a satisfactory explanation. Schlomka attributed the lack of a trans- lational effect to the ships’ low speed during these observations (Abs. 755) and maintained that his measurements in flight had a fair chance of success. In their further correspondence, and despite Einstein’s skepticism, Schlomka de- veloped a detailed program of observations, and discussed the expected results cal- culated for various conditions (Docs. 483 and 486). He also examined earlier similar experiments by Michael Faraday, Piotr Lebedev, and Harold Wilson (Abs. 781). He attributed the negative results of their attempts to their being carried out at ground level rather than in a moving flight system. Schlomka performed his experiments and reported on them at the end of March 1927 (Abs. 809). He also met Einstein in his home on 1 April, and proceeded to test Einstein’s idea in July. But Einstein found the results unsatisfactory.[15]