ELECTRICITY AND MAGNETISM 



needle. When this disk was rotated rapidly, the needle 

 was deflected, or even rotated about its axis, in a man- 

 ner quite inexplicable. Faraday at once conceived the 

 idea that the cause of this rotation was due to electric- 

 ity, induced in the revolving disk not only conceived 

 it, but put his belief in writing. For several years, 

 however, he was unable to demonstrate the truth of 

 his assumption, although he made repeated experi- 

 ments to prove it. But in 1831 he began a series of 

 experiments that established forever the fact of 

 electro-magnetic induction. 



In his famous paper, read before the Royal Society 

 in 1 831, Faraday describes the method by which he first 

 demonstrated electro-magnetic induction, and then ex- 

 plained the phenomenon of Arago's revolving disk. 



" About twenty- six feet of copper wire, one-twentieth 

 of an inch in diameter, were wound round a cylinder 

 of wood as a helix," he said, "the different spires of 

 which were prevented from touching by a thin inter- 

 posed twine. This helix was covered with calico, and 

 then a second wire applied in the same manner. In this 

 way twelve helices were 'superposed, each containing 

 an average length of wire of twenty-seven feet, and all 

 in the same direction. The first, third, fifth, seventh, 

 ninth, and eleventh of these helices were connected at 

 their extremities end to end so as to form one helix; 

 the others were connected in a similar manner; and 

 thus two principal helices were produced, closely inter- 

 posed, having the same direction, not touching any- 

 where, and each containing one hundred and fifty-five 

 feet in length of \vi- 



"One of these helices was connected with a galva- 



241 



