424 Lord Rayleigh on the Theory of Resonators, 



loudness ; otherwise it may happen that a part or the whole of 

 the previously existing sound is extinguished. 



The first case is that of the tuning-fork. In consequence of 

 the comparative thinness of the prongs, the positive and negative 

 sources nearly neutralize each other at a little distance, the air 

 displaced by the advancing faces readily taking advantage of the 

 room afforded by those retreating. Although the fluid moves 

 nearly as though it were incompressible, there is in consequence 

 of the inertia a certain variation of pressure, of which a reso- 

 nator whose mouth is suitably placed can take advantage. 



One of the best examples of this use of a resonator is afforded 

 by a vibrating bar of glass or metal held at the nodes. A strip 

 of plate glass about a foot long and an inch broad, of medium 

 thickness (say ^ inch), supported at about 3 inches from the 

 ends by means of string twisted round it, answers the purpose 

 very well. When struck with a hammer it gives but little sound 

 except overtones ; and even these may be nearly got rid of by 

 choosing a hammer of suitable softness. This is a consequence 

 of the small dimensions of the bar in comparison with the wave- 

 length, which allows of the easy transfer of air from one side to 

 the other. If now the mouth of a resonator of the right pitch * 

 be held over one of the free ends, a sound of considerable force 

 and purity is obtained by a well-managed blow. In this way 

 may be constructed an improved harmonicon, with tones much 

 graver than would be possible without resonators. In the ordi- 

 nary instrument the wave-lengths are sufficiently short for the 

 bar to communicate vibrations to the air independently. 



The reinforcement of the sound of a bell by a resonator in a 

 well-known lecture-experiment is an example of the same mode 

 of action. 



The other part of the theory (the neutralization of a simple 

 source) is not so easily illustrated experimentally, for reasons 

 already referred to. It usually requires more elaborate appli- 

 ances to produce a simple than a compound source; and then 

 there is a certain incongruity in the use of the word simple. The 

 following experiment, however, deserves mention. Suppose that, 

 in the manner just explained, we have obtained a pure sound by 

 the combination of a resonator with a vibrating bar. In this 

 arrangement the mouth of the resonator is the effective source ; 

 and according to theory we should expect that the sound would 

 be again extinguished by the presentation to the first of a second 

 resonator, held in such a position that the bar does not act upon 



* To get the best effect the mouth of the resonator ought to be pretty 

 close to the bar ; and then the pitch is often decidedly lower than it would 

 be in the open. The final adjustment may be made by varying the amount 

 of obstruction. 



