London, South Kensington
6 December 2002
EINSTEIN, Albert. Autograph letter signed ('Papa') to his son Hans Albert, n.p., n.d. [4 March 1927], in German, 2 pages, 4to; envelope. Provenance: Einstein family, and by descent.
CONSIDERATION OF A PROBLEM APPARENTLY INVOLVING A RECORD PLAYER. The nub of the matter is not the magnetic components, for which heavy material is required, but the membrane and its disposition, which needs very light material. The membrane has a very deep free vibration within the acoustic limits. Hans Albert has made things unnecessarily difficult for himself in his derivation of the problem by trying to take induced electromagnetic forces into account: air resistance consequent on the sound production is much more of a consideration. So sound emission will have to be represented, which is already a considerable little problem ('ein ansehnliches Problemchen'). His concerns about eddy currents in the magnets are unfounded. The question is quite different with a spool, because of the considerable magnetic and electrical resistance involved. In conclusion, Einstein suggests there is not much to investigate: the biggest improvement recently has been the use of large, light and rigid records, not requiring a sound-trumpet. The letter concludes with news of an expedition with Hans Albert's younger brother Eduard: they will be in the countryside for a week, and have been making a lot of music.
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