226 



M. Dvorak on Acoustic Repulsion. 



means for observing the acoustic repulsion of resonators. As 

 the method described there is not very sensitive, I have replaced 

 it by the following. The resonators here employed are usually 

 made of stiff drawing-paper covered with gum-arabic, and 

 have the shape of a cylinder, with a little paper tube, hf, at 

 one end (fig. 1, A). This little tube may also be omitted (as 



in B) : in that case the resonator is 'tuned by increasing or 

 diminishing the little opening, fg. Even a cylindrical tube 

 open at one end, C ; may serve our purpose as a resonator. 

 Spherical resonators of glass, D, which a practiced glass- 

 blower can make as light as paper resonators, are excellent. 

 The note of the resonators is determined by gently blowing 

 over the opening or by tapping. 



The resonator is fastened with sealing-w r ax to the end of a 

 light wooden rod, the other extremity of which is provided 

 with a counterpoise of lead, (fig. 2). The centre of the rod 

 has a glass cap, H, w<hich rests on a needle-point. 



The best source of sound is a resonant box of a tuning-fork 

 (fig. 2). The repulsion is so great that it is apparent even with 

 an ordinary brass Helmholtz resonator weighing, with the lead 

 counterpoise, 142 grams*. With every tuning-fork we must 

 first ascertain whether the air in the resonating box vibrates 

 w T ith sufficient energy, because this is not always the case even 

 with accurately tuned boxes. As the elasticity of the different 

 boards which form the elastic system of the box is not equal, 

 their vibrations may hinder the formation of the node at the 

 bottom of the box ; in this case the air on the bottom of the 

 box w r ill vibrate but feebly. We can easily ascertain this fact 

 by accurately tuning the box to the note of the fork, and then 

 observing whether the note is considerably weakened by par- 

 tially covering the opening. If it is not, then the air in the 



* The apparatus represented in fig. 3 may also be used to show acoustic 

 attraction by turning the closed end of the resonator toward the box. 



