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NOTE ON THE DECAY OF MAGNETISM IN BAE MAGNETS. 



By WILLIAM BEOWN, B.Sc, 

 Professor of Applied Physics, Eoyal College of Science for Ireland, Dublin. 



Eead Februarv 24. Published Mauch 12, 1920. 



Section I. 



At a meeting of this Society on December 21st, 1909, the writer read a paper 

 on " Permanent Steel Magnets." Section 1 of that paper dealt with the 

 dimension-ratios of the magnets which were made of the same kind of steel, 

 and Section 2 with the moments of magnets made of steels of various 

 chemical composition. These magnets in Section 2 only are dealt with in 

 the present communication ; and at that time (about ten years ago) they 

 were tested for magnetic moment per gramme at intervals for six months, and 

 the percentage loss in magnetic moment for a period of six months measured. 

 The magnets were then put away from all disturbing influences, and have 

 remained in a vertical position for about ten years, and have now.been re- 

 tested for magnetic moment, with the results given in this note. 



The details of the tests and the heat treatment of the magnets are given 

 on pp. 316-320 of the paper referred to, and the results now obtained are 

 given and compared with the previous results, that is, the percentage loss in 

 the magnetic moment per granmie for each magnet for a period of about 

 ten years. 



The magnets were 8 cms. or 10 cms. long and 0'3 cm. diameter, giving 

 dimension-ratios of 27 and 33, as shown in Table I. 



In Table I the numbers 1 and 6 are left out, as these referred to magnets 

 made of practically pure iron, and were used in the previous paper merely as 

 standards of reference. 



'Scient. Proc, Roy. Dublin Soc, February, 1910, vol. xii, pp. 312-320. 



