120 DOVE ON THE ELECTRICITY OF INDUCTION. 



tion, that it is a substance in which the iron capable of being 

 magnetized does not form a connected continuous whole, a result 

 which accords with the chemical researches of M. Karsten. 



1. Effect of the inversion of the magnetic polarity upon the 

 induced current. 



20. In the foregoing phaenomena an important circumstance 

 has not yet been noticed, a neglect of which would render it im- 

 possible to compare different kinds of iron with each other, — I 

 allude to the effect produced upon the induced current by the 

 inversion of the magnetic polarity. Malleable iron, steel, nickel, 

 and cast iron retain always a greater or lesser portion of the 

 magnetism which is momentarily excited in them ; it becomes 

 therefore important to know, when they are again electro-magne- 

 tized, what relation the metallic bar which is already magnetic, 

 bears towards the magnetizing spiral containing it, whether 

 namely, the remagnetizing produces a like polarity to that al- 

 ready existing in the magnetic bar, or whether a contrary po- 

 larity ensues. The influence which this exerts upon the phaeno- 

 mena of induction was ascertained in the following manner : — 

 When perfect equilibrium as regards the galvanometer had been 

 established between two cylinders, the position of one of them 

 was inverted in relation to its spiral, so that it was magnetized 

 in an opposite direction on again closing the circuit. The dis- 

 turbance of equilibrium which hence arose always showed that, 

 by inversion, the induced current became more powerful. Pre- 

 vious magnetizing by touch gives analogous results to those pro- 

 duced by electro-magnetizing. If iron horseshoes are used 

 which project with one limb into the spirals as far as their point 

 of neutrality, they become converted into tri -polar magnets. 

 Now, as an electrical current of sufficient intensity, when it 

 exerts a magnetizing influence upon a bar of iron so as to cause 

 polarity in a contrary direction to that already existing in the 

 iron, annihilates immediately the polarity which the iron retained 

 from the previous magnetization, and then produces the maxi- 

 mum polarization of which it is capable, the fact here adduced 

 may be expressed by the following axiom: — That metal is pos- 

 sessed of the most powerful inducing action in which the greatest 

 change of magnetic properties occurs*. The indications of the 



• In the construction of magneto-eloctrical machines, it follows directly from 

 this, that in the alternation of the currents there exists a peculiar principle for 

 increasing their power- 



