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XXI. Notes on Magnetism. — I. On the Energy of Magnetized 

 Iron. By Lord Rayleigh, Sec. R.S.* 



THE splendid achievements of the last ten years in the 

 practical application of Magnetism have given a re- 

 newed impetus to the study of this subject which is sure to 

 bear valuable fruit. Especially to be noted are two memoirs 

 recently published in the ' Philosphical Transactions ' of the 

 Royal Society, by Prof. Ewingf, and by Dr. Hopkinsonf, 

 in which are detailed very important data derived from labo- 

 rious experiment, accompanied by much interesting and sug- 

 gestive comment. 



The results of observation are usually expressed, after the 

 example of Rowland and Btoletow, in the form of curves 

 showing the relation between 53 and §, the magnetic induc- 

 tion and the magnetizing force. It may be well here to recall 

 the convention in accordance with which © is measured. At 

 any point in air, the magnetic force is defined in an ele- 

 mentary manner, and without ambiguity, but when we wish 

 to speak of magnetic force in iron, further explanation is 

 needed. The continuity of the iron is supposed to be inter- 

 rupted by an infinitely thin crevasse in the interior of which 

 we imagine the measurement to be effected. If the crevasse 

 is parallel to the direction of magnetization, the force thus 

 found is denoted by §, and is independent of free magnetism 

 on the walls of the crevasse. If, however, the crevasse be 

 perpendicular to the lines of force, there is a full development 

 of free magnetism (-3) upon the walls, and the interior force is 

 now 53, equal to © + 47T-3. In the estimation of § (as well 

 as of S3) the influence of all free magnetism, not dependent 

 upon the imaginary interruption of continuity, is of course to 

 be included. On this account the value of § in the interior, 

 and even at the centre, of a bar of iron placed in an otherwise 

 uniform magnetic field, is greatly reduced, unless the length 

 of the bar be a very large multiple of the diameter. 



Experiment shows that the relation of 53 to § is not of a 

 determinate character. In a cycle of operations, during 

 which § is first increased, and is afterwards brought back to 

 its original value, the induction 53 is always greater on the 

 descending than on the ascending course. This phenomenon, 

 which is exemplified familiarly by the retention of magnetism 

 in a bar after withdrawal of the magnetizing force, is 



* Communicated by the Author. 



t " Experimental Researches in Magnetism,'' vol. clxxvi. part ii. p. 523. 



t " Magnetization of Iron," ibid. p. 455. 



