Intelligence and Miscellaneous Articles. 399 



by agitation, is altered in the opposite direction by extensions and 

 contractions of the wire. 



According to the prevalent views of the nature of magnetism, it is 

 probable that the changes in the length and in the magnetic mo- 

 ment take place nearly simultaneously. If this is correct, when ex- 

 pansions and contractions succeed each other at very short intervals, 

 chauges in the magnetism must occur in a similar manner. 



An iron bar 1890 millims. in length, which makes the longitudinal 

 vibrations of the fundamental note, performs about 1300 entire vi- 

 brations in a second. Hence at the nodal points of the bar the 

 transition from the greatest condensation to the greatest expansion 

 takes place in the 13 1 ft of a second. 



The author has found in fact that, provided the iron is sufficiently 

 soft, even in this case, in spite of the smallness of the elongations 

 and notwithstanding the rapidity of the oscillations, very appreciable 

 variations in the magnetic moment accompany the periodical changes 

 in density. 



A change in the magnetism is most easily proved by the induc- 

 tion-currents which it produces. But if equal and opposite changes 

 in the magnetism succeed each other, alternating induction-currents 

 are produced which cannot be detected by the ordinary galvano- 

 meters. Yet W. Weber was able to demonstrate the existence of 

 such alternating currents by the electrodynamometer, which is in- 

 different to the direction of the currents which traverse it. W. Weber 

 used the above instrument *, which is of his invention, to demonstrate 

 the alternating induction-currents which are produced in an induction- 

 spiral by the periodical motions of the free ends of a transversely 

 sounding magnetic steel bar. I myself have used the same instru- 

 ment to demonstrate the induction-currents which are produced in 

 an induction-spiral by the periodical changes of the magnetic moment 

 on the nodal point of an iron wire surrounded by a magnetizing spiral. 

 A piece of commercial iron wire 1 890 millims. in length was firmly 

 fastened in the middle, so that, when rubbed with a piece of leather 

 coated with powdered resin, it produced the longitudinal funda- 

 mental note, and thus a nodal point was formed in the middle. 

 One half of this wire was almost entirely surrounded by a magnet- 

 izing spiral ; on the other half, at the node, was a short induction- 

 spiral of copper wire, while the rest was free, so that the wire could 

 be rubbed. The short induction-spiral was inserted into a circuit 

 containing a reflecting dynamometer, and the motions of the mirror 

 were observed by means of a scale and a telescope. The magnet- 

 izing current was furnished by two Bunsen's elements. When the 

 rod was strongly rubbed, a deflection was obtained in the galvano- 

 meter of from thirty to fifty divisions. When the induction-spiral 

 was pushed on the free end of that part of the wire which was left 

 uncovered by the magnetizing spiral, no deflection was obtained in 

 the galvanometer. It must thence be concluded that the induction- 

 currents in the first case are not produced by the backward and 

 forward gliding of the magnetic particles of iron, which gliding is 

 least in the middle (that is, at the node) and is most powerful at the 

 * Electrodyn. Maassbest. vol. i. § 16. 



