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Article X. 



X. On the Phcenomena of closed Electro-Magnets, 



By J. C. POGGBNDORFF. 



[From PoggendorfT's Annalen^ vol.lxxxv. p. 147.] 



By closed electro-magnets, the author understands those of 

 the ring form, or of the horse-shoe form with the armature laid 

 on, thus establishing a continuity more or less perfect. Mag- 

 nets of the first class have been hitherto but little examined ; 

 those of the latter, on the contrary, have been often investigated, 

 and more particularly with reference to the amount of the so- 

 called lifting power, as compared with the strength of the gal- 

 vanic current by which the magnetism is excited, by Fechner, 

 Lenz and Jacobi, Joule and CErsted. The results of these 

 physicists are however in part divergent and in part contra- 

 dictory. To the author, therefore, it did not seem superfluous 

 to take up the subject once more, especially as other interesting 

 and hitherto unexhausted questions presented themselves in 

 connexion with it. 



The experiments which he has carried out with reference to 

 this object, corroborate, in the first place, a result already sug- 

 gested by the experiments of CErsted and some of those of Lenz 

 and Jacobi ; namely, that the lifting power increases more slowly 

 than the strength of the current, but with the addition, that this 

 increase with augmented current advances more and more slowly, 

 so that the lifting power approximates asymptotic to a constant 

 value, a value, the absolute magnitude of which depends, of 

 course, upon the nature of the magnet and its keeper. 



In proof of this we will here introduce the results of a series of 

 experiments in which the magnet and keeper were in immediate 

 contact, the strength of the current being measured by the 

 sine-compass. 



