VIBRATION OF FERROMAGNETIC WIRES. 5 



converging lens, was formed in the field of n micrometer ocular. 

 If the wire makes a rapid longitudinal vibration, its amplitude 

 can be measured bv observing- the broadenini»- of the imasfe 

 of the slit. 



The wire to be tested was 21 cm long and 1.5Û mm thick. The 

 magnetizing coil was 30 cm long and wound in 4 layers on a wooden 

 frame and gave a field of 19.82 C.G.S. units due to a current of one 

 ampere. The coefiicient of self-induction of the whole circuit was 

 5.2 X 10^ cm and its resistance 12.9 i^, so that the time of relaxation 

 w^as -l.O X 10~* seconds. 



4. The results of experiments may be summarised as follows : — 



(a) Wires of nonmagnetic metals give no sound by an 

 intermittent or alternate field of any frequency up to 200 

 per second. 



(b) A ferromagnetic wire emits îin audible sound in an inter- 

 mittent or alternate field. 



(c) The pitch of the sound is always the same as that of the 

 make or break in an intermittent or alternate current. 



(d) The amplitude of vibration is in general far greater than 

 the change in length produced by a constant field of such 

 strength that it is equal to the maximum value of the 

 intermittent or alternate field. 



The pitch of the sound was determined by tuning a monochord 

 to the period of the sound and counting the number of beats. 

 From the results above mentioned, we may safely conclude that 

 the sound produced in the ferromagnetics is due to the magnetic 

 chanofe in lenofth of the wire. One make or break of the current 

 forces the wire to accomplish a vibration, and a succession of such 

 series constitutes a sound, the pitch of which is the same as that of the 



