Septkmbeb 21, 1883.] 



SCIENCE. 



393 



inch holes concentric with the disk, the number 

 of holes in the two circles being thirty-two and 

 sixty-Cour respectively. Ou one side of the 

 disk was placed a horse-shoe magnet with its 

 poles vcr}- near the rows of holes ; on the other 

 side were arranged two corresponding induc- 

 tion bobbins. Tlie circuit was completed 

 through a telephone and either bobbin at pleas- 

 ure. L'i>on rotating the 

 disk rapidly, a clear 

 musical sound was pro- 

 duced in the telephone, 

 tlie pitch rising with 

 the rapidity of rota- 

 tion. Moreover the 

 bobbin opposite the 

 circle of sixty-four 

 holes gave the octave 

 above the other, and 

 each gave a note of 

 the same pitch as was 

 produced by blowing a 

 stream of air through 



the corresponding holes. Hence, as a beam 

 of light, focused upon a circle of cqui-dis- 

 tant holes in an opaque disk, is rendered 

 periodically intermittent by the rotation of the 

 disk, and i)roduces a musical tone when fiiUing 

 upon the proper receiving-apparatus ; so the 

 lines of force proceeding from a magnet maj- 

 be rendered periodically intermittent in their 

 action on an induction bobbin by a similar 

 metallic disk, set in rapid rotation ; and the 

 induced currents, arising from the i)criodic 

 change of magnetism in the core of the bobl)iu, 

 produce a musical tone in a telephone, the pitcii 

 depending in both cases only u[)on the num- 

 ber of holes passing in unit time. 



GNETIC CURVES ' 



or in opposite directions through the telephone. 

 In the latter case, an almost perfect neutraliza- 

 tion of currents took place, so that the sound 

 was scarcely audible. 



Non-magnetic metallic disks produce similar 

 musical notes by the periodic modification of 

 the m.agnetic field by means of tiie distortion 

 or bending of the lines of force. The solid 

 parts of the conducting 

 disk deflect the lines of 

 force in the diiection of 

 the rotation ; but upon 

 tlie passage of a hole, 

 they fall back toward 

 tlieir normal position. 

 A periodic movement 

 of the lines of force 

 will, therefore, take 

 place when the disk 

 rotates. Disks of zinc 

 and copper produce a 

 clear musical sound, 

 somewhat less intense 

 than that given by iron under the same con- 

 ditions. Any discontinuity- in the rotating 

 disk recurring periodically will produce cor- 

 responding induction currents in the bob- 

 bins. Thus, V-shapcd notches round the 

 circumference of the disk are quite as effi- 

 cient as the holes in etfecting the requisite 

 modification of the magnetic field. Moreover, 

 it is not necessary that the holes extend en- 

 tirely through the disk. Two disks of zinc, of 

 the same diameter and thickness, were placed 

 together on the same rotating spindle, one 

 l)ierced with a circle of holes, and the otiier not. 

 The combination proved as efficient in produ- 

 cing the sound as the single perforated disk. 



ORSE-SHOE MAGNET. 



EfFECT or SCBEEN OF SHEET IRON. 



FFECT OF UOLE8 TUROUGU TUB IRON SCREEN. 



The experiment was modified b\- so placing 

 the poles of the magnet tliat the same circle 

 of holes passed them in succession. 15v the 

 proper connections, the currents from the two 

 ijobbins were made to pass either in the same 



A sheet of tinfoil, with a circle of small holes, 

 was pasted on the continuous zinc disk. The 

 perforations, extending only the thickness of 

 the tinfoil into the compound disk, constituted 

 a sullicient iliscontiuuity to produce a clear, 



