384 DOCUMENT 333 DECEMBER 1911 "point to point" method (for background to this method, see Phillips 1987, pp. 81-129). Habicht's modification probably consisted of arranging a set of several condensers and charg- ing them at different points of the cycle. In principle this improvement would generate several values of the waveform simultaneously. [3]This phrase and the placement arrow are written by hand. [4]The bifilar loop oscillograph, first proposed by André-Eugene Blondel (1863-1938) (see Phillips 1987, pp. 138-143) and manufactured by Siemens & Halske (see Siemens & Halske 1907, pp. 30-31). The current to be measured passed in a double loop of very thin wire carry- ing a small mirror and placed between the poles of a powerful electromagnet. The mirror al- lowed the deflection of the wires to be observed by means of a light beam. The path of the beam was recorded on a photographic film. [5]A reference to the glow-light tube oscillographs in use at the beginning of the century. The shape and size of an electrical discharge (glow) in a partially evacuated tube varies with the current flowing in it. Observation of the tube in a rotating mirror allowed visualization of the variation of the electrical waveform applied to it (see Lehmann 1909, pp. 1701-1702). [6]HABICHT had demonstrated the "Maschinchen" twelve days earlier before the Deutsche Physikalische Gesellschaft in Berlin (see Doc. 330). [7]Probably a reference to a quadrant electrometer developed by Julius Elster (1854-1920) and Hans Friedrich Geitel (1855-1923) and described by them in Elster and Geitel 1898. Used for measuring weak electrical voltages, this instrument had a moving needle, which was placed between four metallic plates and suspended by a very thin platinum wire. [8]Haber may have attended HABICHT'S demonstration. [9]Marie Curie. [10]The ultra-resistor was later offered as an accessory to the "Maschinchen" (see the edito- rial note, "Einstein's 'Maschinchen' for the Measurement of Small Quantities of Electricity," pp. 51-55). 333. From Max Laue München. Bismarkstr. 22. 27. XII. [1911][1] Lieber Herr Professor Ich habe mich jetzt eingehend mit Ihrer Gravitationsarbeit[2] beschäftigt u. darüber auch im hiesigen Kuolloquium vorgetragen.[3] Ich glaube nicht an diese Theorie, weil ich die völlige Gleichwertigkeit Ihrer Systeme K u. K' nicht zuzugeben vermag.[4] Für das Gravitationsfeld im System K muss doch ein, die Gravitation verursachender Körper vorhanden sein, für das beschleu- nigte System K' aber nicht. Die Frage nach ihm muss doch also sofort ent- scheiden, ob ein wirkliches Gravitationsfeld oder nur ein beschleunigtes Be- zugssystem vorliegt. Trotzdem hat mich die Arbeit sehr interressiert, weil sie die Konsequenzen zeigt, zu denen die Gleichsetzung von Träger und ponde- rabler Masse führt. Ausserordentlich charakteristisch scheint mir auch, dass dabei das Gravitationspotential eine physikalische Bedeutung bekommt, wel- che dem elektrostatischen Potential völlig abgeht. Man könnte ja das erstere