l I N T R O D U C T I O N T O V O L U M E 1 4 appears that today we are much farther away from an understanding of the funda- mental electromagnetic laws than appeared to be the case at the beginning of this century” he asserted in a lecture in October 1924 at the meeting of the Schweizeri- sche Naturforschende Gesellschaft in Lucerne (Einstein 1924p [Doc. 332]). In the same lecture, Einstein observed that, according to Maxwell’s theory, the Earth’s and Sun’s magnetic fields may be the result of “electric currents which flow against the rotation of these celestial bodies.” However, given that such currents can hardly be expected to be present with “sufficient intensity,” he proposed that “the cyclical motion of neutral masses” might be responsible for the production of a magnetic field, despite the fact that neither Maxwell’s theory nor the general theory of relativity “allow us to expect a field produced this way. It seems that nature in- dicates here a fundamental relation that has not been dealt with theoretically” (Doc. 332). In a footnote to the published version of the lecture, Einstein assumed that the hypothesized magnetic property of matter corresponds to a charge of , a procedure that leads to “the correct order of magnitude of Earth’s magnet- ic field,” as he wrote to Bauer (see Doc. 396 and its note 9). Einstein suggests two possible explanations for Earth’s magnetic field (Doc. 332): either bulk matter is not exactly electrically neutral (in which case the classical theory of electromagnetism could explain Earth’s magnetic field), or the magnetic property of bulk matter is due to the (cyclic) motion of electrically neutral matter. Although not explicitly pointed out by Einstein, the former possibility would be a natural consequence of a unified field theory of gravity and electromag- netism, analogous to electrically charged matter producing magnetic fields when in motion. This possibility was raised in the discussion following the lecture. Auguste Piccard remembered that Einstein “proposed the hypothesis that there is a small difference between the two [electric] charges [of the proton and the electron].”[20] This difference would be due to the difference in their mass and would be of the order of , where m is the proton-electron mass difference (i.e., to a good ap- proximation, the proton mass) and K the gravitational constant (i.e., of the order 10–19). According to Einstein’s rough calculations, this tiny charge persisting in bulk matter would suffice to produce the magnetic field of the Earth and the Sun, due to the convective currents of this matter, via Ampère’s law. In Brussels, Piccard started a set of experiments, suggested by Einstein, in- tended to examine whether deionized matter—considered to be completely neutral—has a tiny remnant charge due to the conjectured charge difference between proton and electron. Since the correction, if it exists, should be of the order of 10–19, these experiments required significant skills. Piccard’s letters re- veal that he achieved an extraordinary degree of precision with his experimental m K m K