Page:Light waves and their uses.djvu/23

Rh wave of longitudinal vibrations passes along it, announcing its arrival at the other end by a sound at the box; the time occupied in the passage being perceptibly less than that required for the transverse wave.

The velocity of the wave is in both cases too great to admit of convenient investigation. In order to familiarize the student with wave motion, a number of mechanical devices have been constructed, such as that shown in Fig. 1. Such mechanical models imitate wave motions rather than produce them. They are purely kinematic illustrations, and not true wave motions; for in the latter the propagation is determined by the forces and inertias which exist within the system of particles through which the wave is moving.

The wave model of Lord Kelvin is free from this objection. It consists of a vertical steel wire on which blocks of wood are fastened at regular intervals. It is very essential that these blocks should not slip on the wire, and this end is best accomplished by bending the wire, in the middle of each block, around three small nails, as shown in Fig. 2. For the sake of symmetry two such pieces may be fastened together, with the wire passing between them. Attention may be fixed upon the motion of the ends of the blocks, by driving into them large, gilt, upholstering tacks—a device which adds considerably to the attractiveness of the experiment. The complete apparatus is shown in Fig. 3.

On giving the lowest element a twist, the torsion produced in the wire will communicate the twist to the next element, etc. The twist thus travels along the entire row,