536 Prof. W. Beetz on the Tones produced 



At the same time the beats already mentioned were noticed, two 

 occurring in every revolution. In order to make the rise of 

 pitch distinct to those who have difficulty in discriminating- 

 pitch, it is only necessary to place a sounding-box directly under 

 the rotating fork. If the box be tuned to the pitch of the fork, on 

 slowly turning the latter the phenomenon of interference observed 

 by the brothers Weber is perceived. On turning more rapidly, 

 the tone of fork makes a squalling slide upwards ; but if the 

 sounding-box is now tuned gradually higher, the tone becomes 

 clearer and clearer until the pitch has been raised to the extent 

 above stated. The rise of pitch is likewise heard exceedingly 

 well when a Helmholtz's resonator, tuned to the note of the ro- 

 tating fork, is put into the ear. This phenomenon, however, 

 has nothing whatever to do with the communication of sound by 

 the sounding body to the air, or with its propagation by the latter; 

 for the rise of pitch and the beats are heard as well, or even 

 better, when the ears are stopped and the head is rested against 

 any part of the lathe. The phenomenon is thus entirely objec- 

 tive, and consists in a real increase of the rate of vibration of 

 the fork. It is in fact only another form of Eoucault's pendu- 

 lum-experiment. The vibrations tend to continue in the same 

 plane as that in which they were produced ; they are thus, as it 

 were, transmitted to a thicker bar, and so produce a higher tone. 

 The amplitude of the vibrations at the same time becomes 

 smaller; gradually, however, it increases again, and reaches a 

 minimum* every time that the fork returns to its original posi- 

 tion, or to one differing from it by 180°. It is thus that the 

 beats are produced; the number of them is consequently of course 

 only half as great as that of the beats heard in Weber's inter- 

 ference-experiment. If the fork turns only slowly, the plane of 

 vibration turns with it, and in this case the fundamental tone is 

 heard alone without beats; on turning more quickly, objective- 

 beats soon arise, and the tone rises at the same time, but never 

 to an extent corresponding to the vibrations of a rod whose thick- 

 ness is equal to the longer cross section of the prongs of the 

 fork. 



In confirmation of the correctness of this explanation, I also 

 made experiments with two smaller forks to which I could give 

 a speed of 25 turns per second upon a rotation-apparatus which 

 ran very noiselessly. The prongs of one of the forks {gg) were 

 6 millims. wide and 1*5 rnillim. thick (the thickness being mea- 

 sured in the direction in which the bow is applied to the fork to 

 make it sound); the prongs of the other (cc$) were 1*5 millim. 

 wide and 6 millims. thick. When the forks were set in rotation, 

 the tone of the gg fork rose a fourth, and that of the cc§ fork 

 fell half a tone. In the case of the latter fork, a third and 

 f [Query, a maximum? — Transl.] 



