94 Dr. Faraday's Experimental Researches in Electricity : 



the moving metals, and not by any polarity of their particles. 

 I proceeded therefore to test this idea by different conditions 

 of the cores and the apparatus. 



2657. In the first place, if produced by induced currents, 

 the great proportion of these would exist in the part of the 

 core near to the dominant magnet, and but little in the more 

 distant parts ; whereas in a substance like iron, the polarity 

 which the whole assumes makes length a more important ele- 

 ment. I therefore shortened the core of copper from 6\ inches 

 (2643.) to 2 inches, and found the effect not sensibly dimi- 

 nished; even when 1 inch long it was little less than before. 

 On the contrary, when a fine iron wire, 5^ inches in length, 

 was used as core, its effects were strong ; when the length was 

 reduced to 2 inches, they were greatly diminished; and again, 

 with a length of 1 inch, still further greatly reduced. It is 

 not difficult to construct a core of copper, with a fine iron wire 

 in its axis, so that when above a certain length it should pro- 

 duce the effects of iron, and beneath that length the effects of 

 copper. 



2658. In the next place, if the effect were produced by in- 

 duced currents in the mass (2642.), division of the mass would 

 stop these currents and so alter the effect ; whereas if pro- 

 duced by a true diamagnetic polarity, division of the mass 

 would not affect the polarity seriously, or in its essential na- 

 ture (2430.). Some copper filings were therefore digested 

 for a few days in dilute sulphuric acid to remove any adhering 

 iron, then well-washed and dried, and afterwards warmed and 

 stirred in the air, until it was seen by the orange colour that 

 a very thin film of oxide had formed upon them : they were 

 finally introduced into a glass tube (2653.) and employed as 

 a core. It produced no effect whatever, but was now as in- 

 active as bismuth. 



2659. The copper may however be divided so as either to 

 interfere with the assumed currents or not, at pleasure. Fine 

 copper wire was cut up into lengths of 5£ inches, and as many 

 of these associated together as would form a compact cylinder 

 three-quarters of an inch in diameter (2643); it produced no 

 effect at the galvanometer. Another copper core was pre- 

 pared by associating together many discs of thin copper plate, 

 three-quarters of an inch in diameter, and this affected the 

 galvanometer, holding its needle 25° or 30° from zero. 



2660. I made a solid helix cylinder, three-quarters of an 

 inch in diameter and 2 inches long, of covered copper wire, 

 one-sixteenth of an inch thick, and employed this as the ex- 

 perimental core. When the two ends of its wire were uncon- 

 nected, there was no effect upon the experimental helix, and 



