by Rotating Tuning-forks. 537 



somewhat higher tone could be heard, due, no doubt, to the 

 greatest thickness of the prongs being in the direction of the 

 diagonal of their cross section, and the plane of vibration having 

 to pass through this direction before it could reach that of their 

 width. 



The experiment is much more striking, because simpler, when 

 it is made with a single rod instead of with a tuning-fork. I 

 could not find a rod which would sound loud enough and long- 

 enough when fixed at one end ; but the experiment succeeds ad- 

 mirably with a rod which is free at both ends. A steel rod 1 

 metre long, 16 millims. wide, and 8 millims. thick was hung by 

 one end to a thread which formed a prolongation of the axis of 

 the rod. If I now grasped any one of its nodal points gently 

 between my finger and thumb, and caused it to sound by a blow 

 with a soft hammer, I obtained the note corresponding to its 

 width or to its thickness, according to the direction in which I 

 struck it, — for instance, when it was vibrating with five nodal 

 points in the direction of the thickness, the tone gg ; and when 

 it vibrated similarly in the direction of the width, the tone g. 

 If, however, the rod is struck in the former direction and then 

 turned round through 90°, the tone g is heard distinctly and 

 almost by itself ; while, if it is struck in the latter direction and 

 then turned through 90°, the tone gg is heard as distinctly. 

 Here, then, the plane of vibration actually remains quite constant. 

 If the thread is twisted and then let go after the rod has been 

 struck, both notes are produced as the rod rotates, and in the 

 intervals a droning of other tones corresponding to oblique di- 

 rections across the sectional surfaces. A cylindrical rod treated 

 just in the same way, gave its fundamental note as a continuous 

 steady sound. If the parallelepipedic rod is allowed to hang 

 quietly while sounding, and the ear is carried round it, no 

 changes of tone are perceived ; but if it is hung by a twisted 

 thread held between the teeth, the ears being stopped, the alter- 

 nations of tone and the beats are at once heard. The pheno- 

 menon is consequently here again completely independent of the 

 propagation of sound through the air. 



I attempted to make the phenomenon visible by means of a 

 Wheatstone's kaleidophone, using for the purpose a beautiful in- 

 strument by Konig, with six rods. The experiment succeeds very 

 well with the cylindrical rod, as has been already described by 

 Foucault*. The vibration -figure as seen in the knob, whether 

 the straight line, the ellipse, or the circle, remained quite sta- 

 tionary, thus proving that the plane of vibration was really con- 

 stant. I hoped to see a similar stability in the vibration-curves 

 of the other rods ; but with these such entangled forms were 

 produced that I abandoned all the rest and confined myself to 

 * L'Institut, No. 920. p. 260. Fortschritte der Physik, 1850-51, p. 120 



