104 A. M. Mayer — Researches in Acoustics. 



given by the bar, gives one the means of making many charm- 

 ing experiments in which " sympathetic vibrations " come into 

 play. 



I here describe an experiment which I devised to show the 

 interference of sound in a manner similar to analogous experi- 

 ments in the case of light. The resonant box on which 

 Koenig mounts his UT 5 (1024 v. d.) fork is open at both ends 

 and has a length of nearly a half wave of the sound of the 

 fork. If this resonant box is held with its axis vertical, above 

 an aluminum bar in tune with the vibrating fork, the bar does 

 not enter into sympathetic vibration with the fork, because the 

 sonorous pulses, on reaching the aluminum bar from the two 

 openings of the resonant box, differ in phase by one-half wave- 

 length. But if the axis of the box is held parallel to the axis 

 of the bar, then the sonorous waves reaching the bar have 

 travelled over equal lengths from the openings at the ends of 

 the box, and these waves conspire in their action and the 

 aluminum bar enters into sympathetic vibration. 



As this experiment is an interesting one, I here give details 

 as to the manner of making it. The bar of aluminum has a 

 large surface, having a length of 17 cms and a width of 

 5 cms . The two nodal lines, which are distant from the ends 

 of the bar equal to fths of its length, are drawn on the bar. 

 The bar is supported under these nodal lines on threads 

 stretched on a frame. This frame is of such a height that the 

 under surface of the aluminum bar is 84 cms , or one-quarter 

 wave length, above the surface of the table, so that the vibra- 

 tions of the bar and those of the waves reflected from the table 

 will act together. The upper surface of the bar is covered with a 

 piece of thick cardboard, in which is cat a rectangular aperture, 

 having for length the distance between the nodal lines and a 

 width equal to that of the bar. As this piece of cardboard 

 rest on supports which lift it a slight distance above the sur- 

 face of the bar, the latter, when it vibrates, does not send to 

 the ear the vibrations of the surfaces of the bar included 

 between its nodal lines and its ends, and which vibrations 

 are opposed in phase to those given by the central area 

 of the bar. Thus the sound emitted by the bar is much 

 increased and the experiment rendered mora delicate and im- 

 proved in every way. I have found that the experiment 

 succeeds best when the center of the resonant box is held 



about 58 cms , or, 7 —above the surface of the aluminum bar. 



' ' 4 



This experiment works best in the open air, away from the 

 action of sound-waves reflected from the walls and ceiling of a 

 room. 



