I N T R O D U C T I O N T O V O L U M E 1 4 x l i x field—be performed in the gyrocompass factory of Anschütz-Kaempfe.[19] The re- sults were negative. We also have indications that Einstein may have tried to connect the problem of terrestrial magnetism with the general theory of relativity. In a letter to Einstein in August 1924, Louis A. Bauer, the American expert on terrestrial magnetism, re- called “with much pleasure the very delightful conferences which I had with you at Princeton in 1921 on questions relating to the general theory of relativity and ter- restrial magnetism” (Doc. 298). In the present volume, the matter of terrestrial magnetism first arises in a letter to Einstein on 7 April 1923 (Doc. 5), in which Max Born mentioned rumors that “you have a new theory on the connection of the metric and the electromagnetic field, which should lead to a relation between gravitation and the Earth’s [magnet- ic] field.” The New York Times had indeed reported in its issue of 22 March 1923 that such new work was mentioned in an interview with Einstein while he was in Kantara, Egypt (see Doc. 5, note 17). The possibility of linking the problem of terrestrial magnetism with gravitation- al and electromagnetic field theory did arise, as we have seen, when Einstein con- ceived of testing the relation . About that experiment he wrote to Born some weeks later, on 22 July 1923, that there is a “chance to understand the mag- netic field of the Earth and the Earth’s electromagnetic budget and to verify the view experimentally.” “But we have to wait for the experiment,” he added (Doc. 87). However, as outlined above, the experiments he performed with Her- mann Mark were negative, as Einstein reported in November 1923 to Emil War- burg: “It has now been some time already since the matter of Earth’s magnetism has given a negative result” (Doc. 158). Nevertheless, the topic may have lingered in the back of Einstein’s mind. On 3 May 1924 he mentioned “a cute, simple physical theory” in a letter to Betty Neumann (Doc. 241), and later in the summer it appears that he was testing a hy- pothesis on magnetic forces and accelerated matter (Doc. 303). In her memoirs, Neumann recalled that they once were traveling on a local train directly north-south and south-north in order to observe changes in the behavior of a compass needle (see Doc. 303, note 1). Not surprisingly perhaps, nothing came of that, but a more sophisticated experiment to test the same hypothesis seems to have been devised in August 1924 by Peter Pringsheim (Doc. 303). The negative outcome of his investigation of the affine approach, the difficulties of the overdetermination program, as well as the debate about the reality of light quanta in the wake of the Compton effect may have motivated Einstein to think about some very fundamental modifications to classical physics: “All in all, it fμ βfμ =