To show effects of vibrations in a stretched cord and to investigate relation between frequency , tension and density .It is used for demonstration of stationary waves. it is available in wide-ranging specifications and range. Bench clamp, tuning fork, scale pan and meter scale are the most important parts of this apparatus.Comprising an adjustable length of thin steel rod mounted in a pair of socket terminals near one end and passing through an A.C. energising. It has coil and a permanent magnet .