4 7 2 D O C U M E N T 3 0 3 A U G U S T 1 9 2 4 verkörpert…“[4] Ach ja, es fängt wohl meist im Kopf an!— Und somit wünsche ich Ihnen noch gute Erholung für den Rest der Ferien. Mit besten Grüssen Peter Pringsheim. ALS. [19 145]. There are perforations for a loose-leaf binder at the left margin of the document. [1]Presumably, the aim of the experiment had been to test Einstein’s hypothesis that neutral matter in motion would generate a magnetic field by determining whether a pendulum swinging in the mag- netic field of the Earth would also exhibit torsional oscillations. See also Einstein 1924p (Doc. 332), p. 92, footnote, for Einstein’s hypothesis. Betty Neumann reports in her reminiscences about an experiment that appears to be related to the one described by Pringsheim: “At that period, he [Einstein] was engrossed in one of his most import- ant problems—the creation of a magnetic field by the movement of masses. He wanted to corroborate theory by experiment…one day he asked me to find out the schedule of any train running in a strict north–south direction. It had to be a steam-powered train, not one run by electricity, because any dis- turbance of influence caused by an electric current had to be eliminated.…He handed me the time- table, remarking ‘you know, I am not so good at these figures, they confuse me.’ We boarded the train. He had brought a pocket-compass, which he fastened, by means of wax, to one of the wooden benches on the train. The idea was to watch the behavior of the magnetic needle, which he expected to show different readings on the return trip. The magnetic needle, however, was moving around wildly, thus making it impossible to get any reading at all! He decided then to repeat the experiment the next day. This time, he poured some oil into the compass, thus slowing down the movements of the magnetic needle, sealing the compass itself with wax and again fixing it to the bench. I was to jot down the read- ings which he, by watching the needle, was going to announce every two minutes. He succeeded, for a while, to get the exact positions of the magnetic needle. But as we [read] on, a few soldiers boarded the train, rattling their sabres. He very politely asked them to remove their sabres, and they did. The metallic sabres would have influenced the magnet, as would have any electric current on the train. “On the return trip, however, the readings proved to be identical to those taken on the first trip,— coming and going. Therefore ‘quod erat demonstrandum,’ had not worked out.— He was quite un- disturbed by the negative result of his experiment, and very quietly said that he would have to start all over again” ([90 781], pp. 4–5). Apparently Neumann told the story to her cousin, Hans Mühsam. “I would like to know in which direction you have been going to tackle the problem of the unification of the physical field, which you used to tell me about …” (“Ich hätte gern gewußt, nach welcher Richtung Sie dem Problem der Ver- einheitlichung des physikalischen Feldes, von dem Sie mir öfter erzählten, zu Leibe gerückt sind …” Hans Mühsam wrote to Einstein on 4 August 1942 [38 339]: “Do you remember that, in this context, you once drove by train to Lichterfelde with a little pocket compass in order to test whether the me- chanical acceleration of the train creates a field that corresponds to the magnetic field? The expected effect was not observed, presumably because of the insufficient equipment” (“Erinnern Sie sich, daß Sie einmal—im Zusammenhang mit dieser Frage—mit einem kleinen Taschenkompass auf der Vor- ortbahn nach Lichterfelde gefahren sind, um zu prüfen, ob die mechanische Beschleunigung der Ei- senbahn ein dem magnetischen entsprechendes Feld erzeugt? Der erwartete Effect war nicht feststellbar, wohl wegen der unzureichenden Hilfsmittel” Hans Mühsam to Einstein, 4 August 1942 [38 339]). If the experiment reported by Neumann was an unsophisticated precursor of the experiment de- scribed by Pringsheim in this document, the episode might have taken place during Einstein’s stay in Berlin between his return from Kiel on 27 May and his departure for Geneva on or before 21 July. See Einstein’s reference to a “funny, simple physical theory” in his letter to Betty Neumann of 3 May 1924 (Doc. 241) and his reference to Pringsheim’s experiment in his letter to her of 6 and 8 August 1924 (Doc. 300).