18 .TACOBI ON THE APPLICATION OF ELECTRO-MAGNETISM 



netic fluid, or the currents developed by the influence of the 

 currents, reunite as soon as the soft iron is removed. The 

 coercitive force opposes the union of these fluids or cuiTents, and 

 retains them in the position of equihbrium they have assumed. 

 In soft iron, the effect of induction might be considered general ; 

 the mass, or rather the entire surface, is equally attacked ; in 

 tempered steel the magnetism can fix to such and such spot as 

 is capable of retaining it. It is similar to the electrical condition 

 of isolating substances. The analogy between the figures of 

 Lichtenberg and those of Haldat cannot be misconceived. After 

 this, it would not be astonishing to see the magnetizing of steel, 

 by means of very weak electric forces, under the very conditions 

 which leave soft iron intact, even under the influence of highly 

 enero-etic forces. Four steel needles, of 0°''l in length, and of a 

 thickness of 6'"-001, were fixed symmetrically by their ends 

 to the outer surface of a coil which formed a voltaic circuit 

 conjointly with a magneto-electric heUx wound round a horse- 

 shoe. On breaking or perfecting the circuit in /jr or ^ (fig. 6), 

 a magneto-electric current is constantly obtained, sometimes in 

 the voltaic direction, at times in the inverse one. The magneti- 

 zation of the horse-shoe being veiy weak, on account of the cur- 

 rent di^dding itself between the two hehces, the magneto-electric 

 current of contact or of disjunction, is also weak. Notwithstand- 

 ing this, the needles exposed to the shock of a single disjunction 

 were powerfully magnetized, all in the direction \\ hich had been 

 previously determined, in conformity with the established law. 

 The following table exhibits the magnetic forces acquired. 



Duration of an Oscillation 

 No. Before Magnetization. After Magnetization. 



1. 20"-75 3"-62 



2. 26"-25 2"-72 



3. 18"-25 2"-9 



4. 15"'66 3"-3 



It is still necessary to add, that the needles had been purposely 

 arranged so that the poles would be reversed, and this always en- 

 sued. Moreover, whilst the voltaic current was actually passing, 

 there was no evident magnetism in the steel needles, as was the 

 case in the soft iron wire. 



38. 



The law established above (35), and relative to the magnetic 

 condition of the interior of concavities, is neither confirmed nor 



