A stationary wave is formed when two equal progressive waves are superposed on one another when travelling in opposite directions. About the middle of the nineteenth century a German professor of physics called Franz Melde devised a method of demonstrating transverse stationary waves in a string. One end of a string passes over a pulley and is kept taut by attaching a weight, and the other end is fixed to the prong of a tuning fork. When the fork is bowed to set it in vibration a progressive wave travels along the string to the far end and is then reflected back. If the fork is kept in vibration the incident and reflected waves combine to form a stationary or standing wave and the string is seen to vibrate in a series of equal segments (Fig. 29.1). Instead of a tuning fork, some form of
electric vibrator may be used for the experiment, and the string examined with stroboscope (page 290). By this means it may be made to appear stationary or else vibrate in slow motion.
The points marked N are called nodes and here the string remains at rest; in fact makes no difference to the vibration of the string if it is touched with the edge thin card at any of the nodes. Between each pair of nodes the string vibrates increasing amplitude towards the center, where it reaches a maximum. The can points, A, of maximum amplitude are called anti node.
Fig. 29.2 shows five stages covering half a complete vibration of the string. The diagrams should be compared with the progressive wave shown in Fig. 26.1 particularly with respect to the arrows indicating the movement of the string part Twice during a complete vibration the string is perfectly straight, and at these stag every particle simultaneously passes through its rest position. Furthermore, it will seen from the diagrams that the distance between two successive nodes is equal half a wavelength.