19 4 Dr. A. M. Mayer's 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 ex- 

 periments 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 aluminium 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 aluminium 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 

 aluminium 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 aluminium has a 

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

 5 cms. The two nodal lines, which are at a distance from the 

 ends of the bar equal to §ths 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 aluminium bar is 8'4 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 cut 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 rests on supports which lift it a slight dis- 

 tance above the surface 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, 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 more delicate and im- 

 proved in every way. I have found that the experiment 

 succeeds best when the centre of the resonant box is held 



about 58 cms., or 7 7 above the surface of the aluminium 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. 



