Designed to show the effects of vibrations in a stretched cord and investigate the relationship between frequency, tension and density. In addition the provision of electrical contacts, opened and closed by the vibrating armature, allow the apparatus to be used as high speed changeover switch in. Comprises a thin steel rod armature, mounted in a clamp formed by a pair of 4mm socket terminals, so that its free length may be adjusted as desired. An A.C. energising coil surrounds the armature and a permanent magnet provides the necessary magnetic polarisation. The free end of the armature equipped with a small boss and clamping screw for attachment of the cord and also serves as the moving contact when the apparatus is operating as a changeover switch. The complete apparatus is carried upon a box type base 150 x 80mm.