Improved type, to show effects of vibrations in a stretched cord and to investigate relation between frequency, tension and density. 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 coil and a permanent magnet. The other end of the wire has a small boss with clamping screw for attaching to cord. Two adjustable contacts are also fixed on either side of the armature connected to terminal sockets to provide a quick changeover switch in capacitor charge/discharge experiments etc.