I N T R O D U C T I O N T O V O L U M E 1 4 li setup, which he repeatedly modified in consultation with Einstein. He himself re- alized that this work was a unique feat in experimental physics. The correspon- dence between Einstein and Piccard on various aspects of these experiments continued over several months during the fall of 1924 (see Docs. 338, 343, 377, 401, 405), but the final result was negative: no difference whatsoever could be found between the charges of the proton and the electron (Doc. 405). Einstein appreciated Piccard’s efforts: “I admire your perseverance and am happy with your success […]. You were right in striving for the highest precision, since there is no reason to believe a priori that protons and electrons have equal charges. The negative result that the inner neutral matter has no charge of the order is highly interesting. It does no harm if one hope [of explaining terrestrial magnetism] has to be buried it belongs to a large and good company” (Doc. 407). During the discussion following his lecture in Lucerne, Einstein also remarked that, according to the theory of relativity, the Poisson equation cannot be applied on a large scale (see Doc. 355). Thus, he believed that any unification with gravi- tational theory would lead to small corrections to Maxwell’s equations. This would imply that the magnetic field of the Earth deviates from the Maxwell-Ampère laws. The field would not be exactly perpendicular to the plane of the current (i.e., not exactly parallel to the Earth’s axis of rotation). Several physicists took these ideas seriously and attempted to explore them ex- perimentally. The first to do so was Louis A. Bauer, director of the Department of Research in Terrestrial Magnetism at the Carnegie Institution in Washington, D.C., who had met Einstein in Princeton in 1921. As we learn from his letter to Einstein of 4 August 1924, mentioned above, Bauer now reported in some detail on magnet- ic field measurements showing non-potential (i.e., non-Maxwellian) components (see Doc. 298, in particular note 8). He had found deviations of 3% from the clas- sical potential theory, indicating that the field is, indeed, not parallel to the axis of rotation. Bauer wondered whether Einstein had a theory predicting the angle be- tween the axes, which Einstein did not. In his reply, Einstein repeated the assump- tion made in the Lucerne lecture, namely, that the magnetic property of matter corresponds to a charge of , pointing out again that this gives rise to the cor- rect order of magnitude of the Earth’s magnetic field (see Doc. 396 and its note 9). Another participant in this broadening project was Albert Wigand, extraordinary professor of atmospheric physics at the University of Halle. In October 1924, he reported in detail on atmospheric magnetic field measurements that showed sub- stantial deviations from those calculated according to Poisson’s law. Wigand hoped for more accurate results by carrying out measurements of space charge deforma- tions around airplanes and the “Amerika-Zeppelin” (Docs. 355, 363, 404). m K m K