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Aagnetische Kraftfelder

Nature volume 74, page 583 (11 October 1906) | Download Citation



THIS is a second edition of Prof. Ebert's well known treatise on magnetic fields of force, which first appeared in 1902. The author handles his subject as before with a wealth of illustration, and with a theoretical grasp, which make the book valuable alike to student and teacher. Indeed, the teacher will find in its pages many useful suggestions. Of these is the magnetic vane of Jaumann, depicted on p. 23, vhich recalls the appliance of Petruscheffsky, in which small bar magnet was suspended through one pole, with a counterpoise to make it lie horizontally, and act as a one-pole magnet. Again, the little frame depicted on p. 29 for holding bar magnets during the operation of manufacturing their filing figures on a sheet of glass above them is worthy of notice. The author adopts as a brief synonym for “a point in a magnetic field to which we direct our attention Etoltzmann's term “Aufpunkt,” for which we have no English equivalent. On p. 206 he uses the term “Billiontel” for 10–9 which is surely a slip, since in German, as in English, a billion is 1012, not 109. On p. 54 his definition of unit pole is that it is such as to repel with a force of I dyne a similar pole when at a distance of I centimetre apart in vacuo, whereas hitherto the accepted definition has been when in air. The difference may be unimportant, but it should not pass without challenge. In this edition the author has cut out most of the section upon cyclical systems, and certain deductions of the Maxwell-Hertz equations which were formerly included. On the other hand, he has introduced new matter relating to the electronic view of electricity in its relation to magnetism and to the Zeeman phenomenon. While this part of the book has been shortened, there have been added at the end fresh sections on induction, on the magnetic circuit—a distinctly valuable chapter—and another of lesser merit on dynamo-machines. The author erroneously attributes to Pixii, on p. 359, the invention of the split-tube commutator. What Pixii used m 1832, on the suggestion of Ampère, was the divided mercury-cup familiar to electricians in the primitive motors of Ritchie.

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