x l v i i i I N T R O D U C T I O N T O V O L U M E 1 4 abilities” (Doc. 182). And to Anschütz-Kaempfe he wrote: “I have now drilled my- self so deeply into my scientific problems that—my engine no longer suffices, since the equations at which I have arrived are in fact so difficult to solve that I stand before them like an ox in front of the mountain” (Doc. 184). Nevertheless, in February 1924 we hear that “time passes in diligent work, since I am chewing at a very difficult theory of which I do not know at all whether it leads to the goal” (Doc. 213). He apparently spoke about his efforts briefly at a meeting of the Prussian Academy on 24 April 1924 (see Chronology), and in May wrote to Besso: “Scientifically I am chasing without respite after the quantum problem and really believe I am on the right path—if only it were certain. […] My new efforts are directed at the unification of quanta and Maxwell’s field” (Doc. 253). V It is not clear what the “new efforts” were that Einstein referred to in his letter to Besso. One theme that runs through the entire period covered by this volume is the hope of explaining terrestrial and solar magnetism by some first principle argu- ment that would at the same time allow for its empirical verification. The riddle of terrestrial magnetism had fascinated Einstein since childhood. As he recalled many years later: “A wonder of such nature I experienced as a child of 4 or 5 years, when my father showed me a compass. That this needle behaved in such a determined way did not at all fit into the nature of events […]. Something deeply hidden had to be behind things” (see Einstein 1949, p. 9). Galvanometers and the influence of the terrestrial magnetic field were the topic of his matriculation examination in Aarau (Vol. 1, Doc. 24), and were discussed in Heinrich Weber’s lectures at the ETH that Einstein attended (Vol. 1, Doc. 37, pp. 206–207). Einstein also mentioned terrestrial magnetism in a letter to Lorentz of 23 January 1915 (Vol. 8, Doc. 47), in which he reported on his experiments with Wander de Haas, aimed at measuring the atomic magnetic moment created by electronic currents, as predicted by Ampère’s law. He concluded that, with the positive result, “the reason as to why the magnetic axis and the rotational axis of the Earth nearly coincide has now been found.” Evidence that he continued to think about this topic appears in several letters and reports during the years 1921 through 1923. For example, in the fall of 1921, Ein- stein asked that an experiment capable of simulating the hot mass currents in the Earth’s interior—by rotating a hot metal cylinder and looking for a magnetic