328 Lord Rayleigh : Acoustical Notes. 



success had been already attained in isolating the fundamental 

 tone, only obscured by the accompaniment of octave in 

 unexpected amount. A sensible revival of the fundamental 

 tone ensued when the loads were rotated from their best 

 adjustment though a quarter turn each, corresponding to a 

 lateral shift inwards or outwards of ^ inch. Since the test 

 is of such delicacy, we may perhaps consider the isolation of 

 the fundamental tone to be practically complete. 



The fact remains, and must not be slurred over, that it 

 was not possible by any adjustment of the loads to eliminate 

 the fundamental tone entirely. The residual sound did not 

 come directly through the air from the prongs, but was 

 propagated through the stalk to the resonance-box. It is a 

 little difficult to trace the nature of this residue. Upon the 

 supposition that the vibrations of the various parts of the 

 fork are all in one phase, and of complete geometrical and 

 mechanical symmetry in the construction of the fork, it 

 would appear that some adjustment of the loads must 

 eliminate the fundamental tone. There was, indeed, evidence 

 of actual lack of symmetry, which could not in any case be 

 mathematically perfect. When with the aid of a handle 

 the fork was held horizontally so that its stalk rested upon a 

 wooden edge supported in its turn upon the top of the 

 resonance-box, sound was heard from the box, which varied 

 as the fork rotated round its stalk as axis and in fact nearly 

 vanished in two asymmetrical positions. It would seem that 

 the residual fundamental tone heard in the more normal use 

 may be connected with a lateral movement of the stalk, 

 dependent upon some failure of symmetry. 



As a variation upon the above arrangement the prongs 

 were now bent inwards so that at the outer ends the distance 

 from metal to metal was reduced from 53*5 mm. to 38 mm., 

 a bending intended nearly to represent the effect of the loads. 

 With the 40 gm. loads it was no longer possible to reduce 

 the fundamental tone to silence and, as soon appeared, for 

 this reason that the proper position for the loads w r as 

 unattainable, being that occupied by the metal of the prongs 

 themselves. When smaller (20 gm.) loads were substituted, 

 interior positions could be found allowing the elimination of 

 the fundamental tone to about the same degree of perfection 

 as before. In listening with the Quincke tube to the dying- 

 sound with alternate application and removal of the thumb 

 at the outer end, it was recognized that the low tone was 

 practically gone (thumb on) while tne octave (thumb off) 

 was still fairly audible. About the same displacement of the 



