J oS ee id 
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: 
Michael Faraday, his Life and Works. 163 
of the magnetic action exerted by the currents which induction 
had set up in the copper. We find the second example in the 
fact observed by Foucault, of the sudden stoppage which is like- 
wise experienced by a thick disk of copper set in rotation between 
the poles of an electromagnet the moment the latter is magnet- 
ized. This stoppage is such that it can only be surmounted by 
a considerable effort, and the disk itself becomes very strongly 
heated if the rotation be continued in spite of the resistance it 
meets with. In order that such a heating effect should be pro- 
duced in a mass of such considerable size, and that we should 
this wire is rolled in a coil round a cylinder of soft iron, the 
effect produced acquires great intensity by the fact of the al-, 
ternate magnetization and demagnetization of the iron which 
accompanies the passage and interruption of the current in the 
We all know the advantage that has been taken of this 
combination in the construction of very powerful apparatus. 
isolated conductor by the influence of an electrized body. He 
ascertained, what no one had previously suspected, that the 
i | between the source of electricity 
a great influence upon the effect produced—that, of the vari- 
ous bodies, some facilitated the development of electricity at 
a distance, whilst others completely stopped it. He named the 
former dielectrics; and he proved that these dielectrics, which 
are essentially resins, sulphur, shellac, oils of turpentine and 
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