Ferromagnetic Wires in a Magnetizing Field. 619 



of which Is the same as that of the make and break. If this 

 view be true, the pitch of the sound due to an alternate 

 current must be double that of the sound due to an inter- 

 mittent current for the same number of vibrations of the 

 string alternator, because the magnetic change of length is 

 independent of the direction of the field. By an actual 

 experiment analogous to that of Lissajous, we found this 

 inference to be verified. Our experiments also showed that 

 the magnetic change of length occurs so quickly as to follow 

 a rapid change of magnetization of as much as 150 reversals 

 per second. 



If the frequency be kept constant, the relation between the 

 amplitude of vibration and the maximum field during one 

 complete period of vibration is similar to the relation of the 

 change in length to a steady field. The maximum field used 

 in most of our experiments was 30*3 c.G.S. units. It is also 

 to be observed that if an intermittent or alternate current is 

 passed through a spiral of non -magnetic metals, an audible 

 sound is produced. This is perhaps due to the periodic 

 attraction of the currents flowing through the spiral in the 

 same direction, and is of a quite different nature from the 

 sound just mentioned. 



Gradually varying the frequency of the intermittent or 

 alternate current while the range of the field is kept constant, 

 we observed the singular phenomenon that the amplitude of 

 vibration passed through several maxima and minima. Two 

 marked maxima and minima were observed in the case of a 

 nickel wire. The phenomenon, which was principally due 

 to the longitudinal vibration of the wire, was, to a certain 

 extent, modified by the resonance of the system consisting of 

 a reflecting mirror and two springs attached to it, and to the 

 periodic vibration of the wire due to the magnetic change of 

 length. The position of the maxima and minima were not, 

 however, materially changed by the length of the wire or the 

 tension of the springs. In the case of iron, the magnetic 

 change of length for the same field strength was small, so 

 that the phenomenon was not marked. 



To study the phenomenon specially, we used another 

 arrangement ; the apparatus was the same as that used for 

 the measurement of the magnetic change in length under 

 constant tension. The wire to be tested was about 60 cm. 

 • long and 0*05 cm. thick ; to the extremities of the wire, two 

 copper wires of nearly the same thickness were soldered. 

 It was hung vertically in the axial line of a magnetizing coil 

 80 cm. long, so as to lie nearly in a uniform field, and to its 

 lower end was attached a weight. Near the same end, a 



