186 SEC. 6. SOUND. 



position by means of the set screw fixed at the back, so that the thread makes 

 any desired angle with the longitudinal axis of the tuning-fork. 



It requires but little practice to effect such a tension of the thread that the 

 required number of waves always appears. 



723. Melde's Apparatus for the Combination of Two 

 Thread- vibrations. 



(See Melde's Lehre von den Schwingungscurven, p. 99.) 



Ferdinand Suss, Marburg. 



723a. Melde's Apparatus for the production of Oscilla- 

 tion Curves of two Rectilinear Vibrations on a Strained 



Thread. Ferdinand Suss, Marburg. 



The apparatus consists of two principal parts, one of which is a Lamella, 

 oscillating vertically, fastened on a pedestal, and which is set in oscillation 

 by an electro -magnet. The Lamella itself forms the interference. The 

 other part consists also of a Lamella, which is brought into vibration by an 

 electro-magnet, and the surface of vibration of which can be placed at any 

 desired angle to that of the first Lamella. 



In order to put the apparatus in motion for experiments both parts must be 

 firmly fastened, either to a long, or on two separate solid tables, at such a 

 distance (about 8-12 feet) from each other that the thread when strained 

 measures 3 to 3'5 meters in length. (The thread can, of course, be shortened 

 at pleasure, or replaced by one a few meters longer; but for class experiments 

 the length stated appeared to be most practical.) The thread is fastened to 

 Lamella I., and passed through the hole in Lamella II. towards the peg by 

 which the correct tension is effected. 



The white points on the (red) thread serve for facilitating the better obser- 

 vation of the curves, and, in order to render them more prominent, the black 

 screen is placed behind the oscillating thread. The screen has on one side 

 small holes, which are hinged in the hooks of the frame of Lamella I. ; when 

 the screen is expanded, the foot of it is screwed firmly by a vice-pin to the 

 opposite table. 



Two (large-sized) chrome elements, the strength of which may be easily 

 regulated, will serve best as electro-motors. In the present case two chrome 

 elements were employed, each with tw r o carbon plates, and a zinc plate, of 

 18 cm. in height, and 6 cm. in width, joined together one behind the other. 



When newly charged, it was only necessary to dip the zinc plate from 1 to 

 1'5 cm. into the acid, in order to obtain the requisite strength of electric 

 current. Both elements and the wire spirals of both magnets are, of course, 

 united to form one circuit. As already observed, Lamella I. serves at the 

 same time for interference, for which purpose an attachment screw is fixed at 

 its reverse end, which is intended to be connected with the Avire leading to the 

 battery. To the attachment screw, which is in connexion with the mercury 

 bowl, a wire is fastened, which is to be connected with the first attachment 

 screw of the wire spirals, if the proportion of the numbers of the vibrations of the 

 two Lamellae are from 1:1 to 1:2 (accord, octave). If Lamella II. is to 

 perform three or four vibrations in the same tune in which Lamella I. makes 

 only one vibration, then only one wire spindle of Lamella I. is set in motion 

 by fastening the wire coming from the mercury bowl in the centre attach- 

 ment screw. The current must always be so powerful that the interference, 

 whose rough displacement is effected by raising and lowering the platinum 

 pin, and the more minute displacement by turning the mercurj 7 bowl, can be so 

 placed that the shutting off of the current is effected as quickly as possible, 

 as upon this the purity of the figures chiefly depends. 



