326 



Lord Kayleigh : Acoustical Notes. 



fr 



concentrated at the ends of the prongs. Then it' the fork be 

 free in space, these ends can move only backwards and 

 forwards along the line joining them. The question we have 

 to ask is — does the stalk remain at rest ? A little consi- 

 deration makes it fairly clear that in the case of parallel 

 prongs the answer is in the negative. As the prongs approach 

 one another the curvature of the bend is increased and the 

 stalk moves along its length outivards > i. e. away from 

 the prongs *. Similarly half a period later the opening of 

 the prongs is accompanied by an approach of the stalk. 

 Under these conditions if the stalk be brought into contact 

 with a sounding-board, a motion of the first order is commu- 

 nicated and the principal tone is heard. 



It is evident that the effect to be expected Fig. 4. 



when the prongs are parallel may be compen- » 

 sated by a suitable permanent bending of the ( 

 prongs inwards, or what comes to the same 

 by a suitable loading on the inner sides. 

 The motion of the stalk during the vibra- 

 tion is then composed of two parts which 

 have opposite signs — the one already con- 

 sidered depending on the variable curva- 

 ture at the bend, the other on the obliquity 

 of the prongs to the line of motion at the 

 ends. It would appear then that by this 

 adjustment it should be possible to secure 

 that the stalk remains at rest, so far as 

 motion of the first order is concerned. It is 

 assumed that everything is symmetrical, so 

 that the stalk, if it moves at all, does so along 

 its length and (in view of its dimensions 

 relatively to the wave-length of vibration in 

 steel) practically as a rigid body. 



For the purposes of the experiments a 

 large fork was constructed by Mr. Enock. 

 The U was from a single length of steel 

 60 cm. long and of section 1*275 cm. square. 

 The prongs were parallel, 5'35 cm. apart 

 (inside measurement), and the stalk was 

 attached by brazing (fig. 4). 



In its unloaded condition it gave 12<S 

 vibrations per second and could be screwed 

 to a resonance-box appertaining to a large 

 fork by Konig of the same pitch. When 



* Somewhat as if by a violent local bending at the middle of the U 

 the prongs were brought into contact throughout their whole length. 



