HANDBOOK OF ACOUSTICS - online book

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ON THE VIBRATIONS OF STRINGS.               91
D, E, F (fig. 46), which seem to be at rest are termed "nodes" or " nodal points,'" and the vibrating portion of the tube BD, CE, or EF, between any two successive nodes is called a "ventral segment"
To understand how these nodes are formed, let a c, fig. 47 (1) represent a string similar to that just referred to. By jerking the end a, a hump a b is raised, which travels to the other end. In fig. 47 (2) this hump has passed on to be. In fig. 47 (3) it has been re­flected, and is returning to the end a, but on the opposite side. While this has been going on, let us suppose another impulse to have been given, so as to produce the hump a b, fig. 47 (3). Now the hump b c is about to pass on to a, and in so doing, the point b must move to the left, but the hump ab is about to travel on to c, and in so doing must move the point b to the right. The point b, thus continually urged in contrary directions with equal forces, while the humps pass one another, remains at rest. Suppose that the hump takes one second to travel from a to c and back again: then it is evident that if an impulse is given every half second, the above state of things will be permanent, and the two parts ab, be will appear to vibrate independently of each other, fig. 47 (4), the point b forming a node. A little reflection will show that, on the same supposition, if the impulses follow one another at intervals of one-third of a second, two nodes and three ventral segments will be formed, and so on. When therefore a string vibrates in 2, 3, 4 segments, each segment vibrates 2, 3, 4 times as rapidly as the string vibrating as a whole.
A tuning-fork may be used with great advantage, in setting up these segmental vibrations. One end of a silk thread is fastened to one of the prongs of the fork, the larger the better; the other end being either wound round a peg, or after passing over a pulley,