Intelligence and Miscellaneous Articles. 507 



MAGNETIZATION OF HOLLOW AND SOLID CYLINDRICAL CORES. 



BY H. DU BOIS. 



The vexed question of hollow v. solid cores is once more dis- 

 cussed in this note. Von Peilitzsch's well-known result that the 

 magnetic moment of short hollow cores is hardly inferior to that 

 of solid ones (other circumstances equal and saturation not being 

 approached) has been elegantly explained by Prof. Silv. Thompson 

 in a general qualitative way by the simple remark that the external 

 air-resistance is approximately the same in both cases. There is 

 no need whatever, therefore, for the assumption that the central 

 fibres of cores are less easily " reached " by temporary steady 

 magnetization than those near the skin : such a state of things 

 would only be proved if the same result were obtained wdth hollow 

 endless rings, which is improbable. 



Von Feilitzsch's experiments having been lately repeated and 

 extended by Prof. Grotrian (Wied. Ann. 1. p. 705, 1893), the 

 author seized the occasion for attempting to go into the matter 

 somew 7 hat more fully. The particular case of a cylinder being one of 

 those in which the modern " magnetic circuit " notion does not 

 lead up to quantitative results so well as the more orthodox theory 

 of induction, the latter had to be applied, and immediately yielded 

 the following general rule : — 



If for cylindric or prismatic bodies of given length and any 

 shape of section the " demagnetizing factor " be proportional to 

 the latter and not too small, then the magnetic moment will be 

 practically independent of the section for fields which are not so 

 strong as to produce an approach towards saturation. 



It is further shown that both premisses of this rule apply to 

 thin hollow cores, for which it is thereby easy to draw the curves 

 of magnetic moment and impressed field up to thorough satura- 

 tion for any given dimensions. An illustrative diagram of such 

 curves is given, which presents exactly the same aspect as the 

 experimental diagram of Prof. Grotrian 's paper. Finally, the 

 question of hollow cores for dynamos is touched upon. Signor 

 Ascoli has since independently criticised Prof. Grotrian's results 

 much on the same lines. — Wiedemann's Annalen, li. p. 529 

 (1894). 



MOLECULAR ENERGY OF GASES. A CORRECTION. 



On page 420, line 9, the word " every " is wrong. It is only 



because collisions are occurring in every direction that on the 



average the original difference of energy diminishes with each 

 collision. — 0. J. L. 



