404 



Mr. R. Shida. 



" Experimental Determinations of Magnetic Susceptibility and 

 of Maximum Magnetisation in Absolute Measure." By 

 R. Shida, Thomson Experimental Scholar, University, 

 Glasgow. Communicated by Sir W. Thomson, F.R.S. 

 Received October 10, 1882. Read November 23, 1882. 



[Plates 9—16.] 



The fact that there exists a limit to the magnetisation of a soft iron 

 bar was first demonstrated by Joule, who, in 1840,* made a number 

 of experiments on the sustaining power of an electro-magnet, and 

 showed that when the current in the exciting coil is made stronger 

 and stronger, that power tends to a certain definite value, or in other 

 words, the magnetisation of the iron core attains a maximum. 



In 1861, an interesting research on the magnetic properties of iron 

 was made by Thalen, who determined, among other things, the 

 magnetic susceptibilityf of different specimens of soft iron in absolute 

 measure for the first time. The units of length, mass, and time 

 employed by Thalen were respectively a millimetre, a milligramme, 

 and a second. 



Joule and Thalen were followed by several, most of whom, however, 

 made experiments without giving the results in absolute units ; but 

 amongst the few who have not overlooked the importance of such a 

 system of units, Rowland made by far the most important investiga- 

 tions upon the subject. He determined not only the magnetic 

 permeability or susceptibility of certain so-called magnetic bodies, 

 but also the maximum magnetisation of those bodies in absolute units, 

 using the metre, the gramme, and the second as the units of length, 

 mass, and time. 



The method of Thalen and that of Rowland are essentially the same, 

 inasmuch as they depend upon the same electrodynamic principle, 

 that an electric current induced in a closed circuit due to sudden 

 creation or disappearance of magnetic lines of force, is proportional 

 to the number of lines of force thus introduced or withdrawn, cutting 

 the circuit. But one notable difference of the two methods lies in the 

 fact that the one used ellipsoids or cylindrical rods of great length, 

 while the other chiefly used rings or endless rods to experiment upon. 

 The chief advantage of an electromagnetic method such as the above, 

 is, as has been remarked by Sir William Thomson in his paper on the 

 " Electrodynamic Qualities of Metals, Part VI, "J the ease and rapidity 



* Joule's Collected Papers, page 34, from " Sturgeon's Annals," vol. v, page 187. 

 f Sir William Thomson, " Papers on Electricity and Magnetism," p. 472. 

 X "Phil. Trans.," 1876, p. 693. 



