=345. Quality.=--The reason for the _differences in tone quality_ between notes of the same pitch and intensity as produced, _e.g._, by a violin and a piano, was long a matter of conjecture. Helmholtz, a German physicist (see p. 397) first definitely proved that tone quality is due to the _various overtones_ present along with the fundamental and _their relative intensities_. If a tuning fork is first set vibrating by drawing a bow across it and then by striking it with a hard object, a difference in the _quality_ of the tones produced is noticeable. It is thus evident that the manner of setting a body in vibration affects the overtones produced and thus the quality. Piano strings are struck by felt hammers at a point about one-seventh of the length of the string from one end. This point has been selected by experiment, it having been found to yield the best combination of overtones as shown by the quality of the tone resulting. [Illustration: FIG. 337.--Chladni's plate.] [Illustration: FIG. 338.--Chladni's figures.] =346. Chladni's Plate.=--The fact that vibrating bodies are capable of many modes of vibration is well illustrated by what is known as Chladni's plate. This consists of a circular or square sheet of brass attached to a stand at its center so as to be held horizontally. (See Fig. 337.) Fine sand is sprinkled over its surface and the disc is set vibrating by drawing a violin bow across its edge. The mode of vibration of the disc is indicated by the sand accumulating along the lines of least vibration, called _nodal lines_. A variety of nodal lines each accompanied by its characteristic tone may be obtained by changing the position of the bow and by touching the fingers at different points at the edge of the disc. They are known as Chladni's figures. (See Fig. 338.) [Illustration: FIG. 339.--Manometric flame apparatus.]