294 H. E. J. Gr. du Bois on Magnetization in 



evidence for the continual increase of the Induction, pro] :- 

 tional to the square root of the weight supported. However. 

 I cannot qnite agree with the author's way of calculating the 

 curve of magnetization*. Quite recently Messrs. Ewing and 

 Low t published the final results obtained with their ingenious 

 ''isthmus'"' method, vrhich disprove the existence of a limit to 

 ©, the induction having been forced up to over 45.00'J C.G.S. 

 The authors further conclude that the magnetization remains 

 sensibly constant throughout the extensive range of mag- 

 netizing field 2000 to "20,000 C.G-.S.j vrhich they were the 

 first to apply. 



§ 2. Mainly vcrth a view to check these results by an alto- 

 gether different method, and to obtain more accurate data 

 than the isthmus method, as developed at present, appears 

 capable of supplying. I undertook the --::' sriments to be de- 

 scribed. Excepting Mr. BidwelTs 585 C.G.S-, fields over a 

 few hundred units hardly appear to have been applied in 

 coils : I therefore began with a powerful coil giving up to 

 130C C.G.S. Thence upward to 13,000 an optical method, 

 1 ased on Kerr's phenomenon i. was used. I have also studied 

 the effect of temperature with these strong fields ; considerable 

 interest having been lately attached by various authors : 

 theoretical thermomagnetic speculations : and thermomagnetic 

 motors having been proposed by Thomson-Houston. Schwe- 

 doff. Stefan. Ellison, and others § . In conclusion, the important 

 part played by magnetization in affecting the properties of 

 matter is discussed. 



Throughout this paper I shall as usual denote by: ^>. mag- 

 netic intensity: 5$. induction ; when the values of these quan- 

 tities inside the metal are considered, they are denoted by (^ 

 and W; % magnetization: D. density: (§=3/D 3 specific 



* The magnetic intensity § in the thin air-films between the halves of 

 the ring evidently equals IB', the induction in the iron. Now the tractive 

 force per unit section is equal to the tension in the lines of force 



(for notation, see § 2). This adaptation of Maxwell's formula bv B jw- 

 land. Mr. BidwelT<Troe. Boy. Soc xl. p. 192, 1 — .- zzis leariy erro- 

 neous,"' though I fail to see why . He adopts the formula 



the missing term Q^/Stt appears, from the action-at-a-distance point of 

 view, to correspond to the mutual electrodynamic attraction of the two 

 half-coils, neglected bv Mr. Bidwell. 



- E~ lz t - arid L:~. Phil. Trans, clvxx. :: i21 1889). 



j See du Bois, Wied. Ann. xxxiv. p. 25 (1690): transl., Phil. Mag. 

 ante, p. 253. the paragraphs of which will e refen ed ix as he. cit. 



§ See Stefan, TTien. Berickt = :. i.y 70 1888). Kittler, Hand- 

 buck der EleldroUchwl:, ii. p. 47 (1890). 



