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XXVI. 



PERMANENT STEEL MAGNETS. 



By W. brown, B.Sc, 

 Professor of Applied Physios, Royal College of Science for Irelanrl. 



[Read DECuMBiiR 21, 1909. Ordered for Publication .Tanuarv 11. Published Febeuaky 21, 1910.] 



Section 1. 



The first important experiments ou periuaiieut steel magnets — since tlie 

 publication of Gauss's paper on Terrestrial Magnetism in 1832 — were made 

 by T. Gray^ in 1878, who gave the results in absolute e.g.s. measure. The 

 magnets he used were 5 cm. long, and O'l cm. diameter, or about 60 

 diameters long ; and the magnetic moment per gramme varied from 

 62 to 79. 



In 1882 L. M. Olieesman^ published the results of some important 

 experiments on magnets made of English silver steel, wliich were hardened 

 by stretching; he found that the best retentivity was obtained with a 

 magnet of dimension ratio^ (length divided by diameter) 41 ; the diameter 

 being 0'128 em. 



In 1885 T. Gray* published the results of some determinations of the 

 horizontal component of the earth's magnetic field, and at the end of his 

 paper a table of values is given for glass-liard steel magnets. By plotting 

 from this table tlie dimension ratio of the magnets against the magnetic 

 moment per gramme, we find for the kind of steel he used, that the best 

 length of magnet would be 10 cm., or dimension ratio 30, and magnetic 

 moment per gramme 40. 



In 1887, the author" published the results of some tests of steel magnets, 

 where, among other things, an attempt was made to obtain the most effective 

 dimension ratio for a given diameter of magnet. 



'Phil. Mag., vol. vi., 1878, p. 321. 



^Wied. Anna!. Bd. xv., 1882, p. 204. 



3 In the Phil. Mag., vol. xvii., 1909, p. 733, Professor S. P. Thompson and E. W. Moss deHne 

 dimension-ratio as the length of the magnet divided by the square root of its cross-seetional area. 

 By this method, magnets whose cross-sections are round, square, or oblong can be compared. 



iPhil. Mag., vol. xx., 1885, p. 497. 



6 Phil Mag., v<d. xxiii., 1887, pp. 293 and 420 ; vol. -x.tvii., 1889, p. 270, 



