224 ELECTRODYNAMICS OF MOVING BODIES p. 27).[3] In addition to ponderable matter or in its absence, the ether provides such a medium.[4] Electricity and magnetism, rather than being the substantive entities that produce such states,[5] are only "Namen" for certain derived concepts (ibid., p. 223). They are convenient and colorful embellishments of the theory that add nothing to its physical content (ibid., pp. 223, 225). Einstein expressed doubt about the role of "des Namens 'Äther'" in electromagnetic theory. His skepticism is directed primarily at Hertz's concept of the ether as a me- dium with a state of motion (see, e.g., Hertz 1892, pp. 256-257) [6] it is not clear whether it extended to the ether concept itself. Einstein attributed fundamental sig- nificance to the concept of "elektrische Massen." Hence, he regarded electric cur- rents as motions of such charges in empty space, rather than as the "Verschwinden elektrischer Polarisation in der Zeit." His views show his acquaintance with "sub- stantive" theories of electricity and magnetism (see note 5), as well as with current investigations of charged particles.[7] In many respects, the view of electrodynamics sketched in this letter (Doc. 52) is strikingly similar to that of Lorentz (see Lorentz 1892, 1895).[8] Einstein concluded that "Strahlungsversuche" were needed to decide between the two viewpoints he contrasted. His next letter (Doc. 54) mentions an idea for experi- mentally investigating the influence of motion relative to the ether on light prop- agation in transparent bodies and a related theory he was developing. His physics professor showed no enthusiasm for this work, but a reading of Wien 1898 encouraged Einstein to write to Wilhelm Wien about his ideas (see Doc. 57). In this paper, Wien discussed thirteen experiments bearing on "die translatorische Bewegung des Licht- äthers," including the Michelson-Morley experiment.[9] About this time, Einstein also mentioned his intention to read Helmholtz 1897 (Docs. 57, 58). [3] In his Introductory Survey (ibid., pp. 1-31), Hertz used "Polarisationszustand" to denote the same concept he had denoted by "Kraft" in his earlier papers (see p. 30). [4] While Hertz sometimes used the phrase "Polarisationszustand des Raumes" (e.g., pp. 28, 30), he made it clear that, if space could be emptied of the ether, it would be incapable of sustaining such states (ibid., p. 28). [5] This "substantive" concept of electricity, particularly as developed by Wilhelm Weber (1804-1891), was widely accepted by German– speaking physicists. It was taken for granted in the lectures on electricity by H.F. Weber, as indicated by Einstein's notes (see Doc. 37, e.g., pp. 178-179). [6] For a much later comment on Hertz's electrodynamics, see Einstein 1979, p. 32. [7] Docs. 52 and 79 indicate Einstein's fa- miliarity with the theory of ions ("ion" was often used generically at this time to denote all charged particles). Doc. 111 shows his strong interest in Lenard's experiments on cathode rays. See also the editorial note, "Einstein on Thermal, Electrical, and Radia- tion Phenomena." [8] Lorentz is not mentioned, however, until Doc. 131. For a much later discussion by Ein- stein of the historical role of Lorentz's theory, see Einstein 1953a, pp. 35-39. [9] Einstein later stated that, while a stu- dent but before he became familiar with the Michelson-Morley experiment, he devised an experiment that was similar to it in principle (see the report of his 1922 lecture at Kyoto University in Ishiwara 1971, p. 79 see Hirosige 1976, p. 54, for an account in English). A light ray is split into two by means of mirrors, one ray traveling in the direction of the earth's motion and the other one in the opposite di- rection. Any difference in the energies of the two rays is detected by thermoelements.