6 JACOBI ON THE AI'l'LIC ATION OF F.I.ECTRO-M AGXETISM 



which gives uniformity to the course of the magnetic machine, 

 might be considerably weakened by employing hollow tubes (18). 



29. 

 The free magnetism of the inner surface of hollow tubes 

 magnetized by an electric helix is very feeble, although the 

 magnetization of the external surface is very energetic. 1 shall 

 return to this subject subsequently, but shall confine myself 

 at present to the remark, that this magnetism does not appear 

 to belong properly to the inner surface, but that it should 

 be attributed to the external layer which acts at a distance, 

 on the testing needle, or on the iron filings which have been 

 introduced into the interior (36). If we employ a hollow tube for 

 the armature of a horse-shoe, the magnetism of the interior 

 is much more decided ; but the distribution at the surface is 

 effected in so complicated a manner that I have not yet been 

 able to account in any Avay for it. If we break the voltaic cir- 

 cuit which serves to magnetize the hor?e-shoe, the armature re- 

 mains adherent to the poles ; unless it is a hollow tube, every 

 trace of free magnetism disappears. When the armature is 

 solid, it still retains some traces of magnetism, which appear 

 to be diffused rather at the lower surface than at the upper part. 

 The force with which the armature still adhered, after ha^dng 

 interrupted the voltaic circuit, was for the cylinder D of one line 

 in thickness ll|lb., and for the solid cylinder only 'Jj lb., their 

 own weight included. These numbers are the arithmetical 

 means of five closely related experiments. 



30. 



A number of interesting questions are naturally connected 

 with researches of this kind, among Avhich, those respecting 

 the state of magnetic saturation of which soft iron is capable, 

 occupy an important place. It is customary to use the expres- 

 sion " magnetized to saturation," chiefly Avhen it relates to the 

 rendering of steel magnetic. But can such a state also exist in 

 soft iron magnetized by {he influence of currents, whose energy 

 and quality may be increased at will ? Is there a limit above 

 which magnetic development cannot be forced? In fact we 

 may expect from experiments undertaken on this subject, bril- 

 liant and fertile results standing in closer connexion with the 

 essence of these j)roblematic forces. But whoever has devoted 

 himself to expensive and wearisome researches of this kind, will 



