CLOSED ELECTRO-MAGNETS. 301 



weaker current. If, however, the reversion of the magnetism be 

 continued, we always obtain these strengthened currents, which 

 considerably surpass those obtained from the open magnet. 



It does not appear that any application of this has been made 

 in the construction of electro-magnetic apparatus. All apparatus 

 of this description seen by the author possess either a straight 

 electro- magnet, or, as that of Neef, a horse- shoe one, which is 

 opened and closed by a moveable keeper, but does not remain 

 permanently closed. From the reversion of the current and the 

 application at the same time of a closed electro-magnet, the most 

 powerful inductive effects are however to be obtained. 



The easy reversion of the residual magnetism by a current far 

 feebler than that which produced it, is a proof of its great mo- 

 bility, and does not appear to corroborate the view of Poisson. 

 In this respect it presents a remarkable contrast with the per- 

 manent magnetism, even in soft iron. 



If an open horse-shoe magnet of soft iron, which possesses a 

 certain degree of permanent polarity, be magnetized by a gal- 

 vanic current in the opposite direction, it can be so arranged, 

 that the iron, while the current continues, may possess the 

 reversed polarity, but on interrupting the current will spring 

 back to its former condition. The strength of the current is 

 the only thing of importance here ; if this be too great the per- 

 manent polarity will also be reversed. 



A similar result was obtained some time ago by the author 

 with one of Logeman^s magnets. A current being sent round 

 the limits of this magnet, which consist of very hard steel, its 

 polarity during the continuance of the current was reversed ; but 

 where the latter ceased, the original polarity, although much 

 weakened, again appeared. 



These observations show, that in a magnet two opposite pola- 

 rizations, or two reversed series of Ampere's currents, may sub- 

 sist simultaneously. 



The foregoing statements with regard to a closed horse-shoe 

 magnet are also applicable to the case when the continuity is 

 perfect, or to the case of an iron ring. This might indeed be 

 surmise, but is nevertheless remarkable. 



