DOCUMENT 118 SEPTEMBER 1908 135 ALS. [6 188]. [1]See Bucherer 1907b. Bucherer's apparatus consisted of a charged capacitor placed in a uniform magnetic field parallel to its plates. In the middle of the capacitor a radioactive source emitting Becquerel rays (ß-rays) was placed. The total force in the direction perpendicular to the plates on electrons leaving the source is determined by the magnitude of the electric field between the plates of the capacitor, the magnetic field, the direction with respect to the mag- netic field in which the electrons move, and the speed of the electrons. For the electrons to es- cape from between the plates, which are very close together, the net force in the direction per- pendicular to the plates has to be zero. Thus for a given combination of electric and magnetic field strengths and direction of motion only electrons with a speed determined by these param- eters can escape. After leaving the capacitor the electrons are deflected by the magnetic field only and registered on a cylindrical photographic plate surrounding the capacitor. From the speed of the electrons and their deflection Bucherer calculated their charge to mass ratio, using two different theoretical frameworks (see below). See Goldberg 1968, chap. 2.A, and Miller 1981, secs. 1.9, 1.11, 7.4.1-7.4.3, and 12.4, for historical discussions of Bucherer's work and other determinations of the charge-mass ratio of electrons. See also Laub 1910, sec. IIc, and Lorentz 1922, chap. 7, for critical discussions of experiments on the charge-mass ratio. [2]Here Bucherer uses the terms "Relativprincip" and "Relativitätsprincip" to denote what he calls the "Lorentz-Einstein theory." (Note a different usage later on in this letter, however.) Like many contemporaries, Bucherer made no fundamental distinction between Lorentz's electron theory and special relativity. Initially, Bucherer did not believe in Einstein's theory (see Doc. 47 for more details see also the last paragraph of Doc. 119). See Bucherer 1908b, the paper in which Bucherer published the results of his experiments, and which has the term "Lorentz-Einsteinsche Theorie" in its title, for more on Bucherer's views on Lorentz's and Einstein's theories (p. 756). [3]The current in the solenoid that generated the magnetic field. [4]Bucherer's results, as summarized in the table above, occur in the same form in Bucherer 1908b. A later, more detailed publication (Bucherer 1909) presents slightly different values (which are still in support of the "Lorentz-Einstein theory"). [5]See Bucherer 1906, 1907a, for Bucherer's principle of relativity. It postulates that for any two interacting electromagnetic systems in relative motion, each system experiences the same force as it would according to Maxwell's theory if it were at rest in the ether while the other system is moving relatively to it. [6]Ebenezer Cunningham (1881-1977) see Cunningham 1908, which was a reaction to Bucherer 1908a. See also Doc. 119 and Bucherer's reply to Cunningham (Bucherer 1908c) for Bucherer's maintaining of his own point of view. The Bucherer-Cunningham discussion is an- alyzed in Goldberg 1968, chap. 2.A.3. 118. From Max Planck [Hotel Bellevue,] Axalp 8. IX. 08. Verehrtester Hr. Dr! Seit 14 Tagen weile ich hier mit meiner Familie [1] doch ist es sehr schwie- rig Ihnen einen bestimmten Ort u. Zeitpunkt für ein Zusammentreffen vorzu- schlagen, da das Wetter einen gar zu maßgebenden Faktor in allen unseren Plänen bildet. Das einzige Sichere, was ich sagen kann, ist, daß ich Sonn- abend d. 12. Abends in Brienz, Hotel du Lac, sein werde, da wir am 13. früh die Heimreise über den Brünig antreten. Wäre es aber nicht praktischer, wir