Useful methods of determining elastic constants employing diametrically–loaded disc and ring specimens have been proposed and applied by Durelli and Ferrer. Young's modulus may be measured from tests on a suitably dimensioned ring specimen. Subsequently Poisson's ratio may be found using a disc. The application of the methods described by the authors does not suit the case where the specimens are small and of stiff materials such as metals. In such a case sensitive means are required for measuring change in diameter with light loading. The paper indicates how this can be carried out for an aluminium alloy. The theory relating to the use of the disc for determining Poisson's ratio has been generalised to allow for measurement of change in length over part or all of a diameter. Poisson's ratio of the aluminium alloy was determined with a disc specimen 0.874 in (22.2 mm) diameter, vibrating wire strain gauges on either side of the specimen indicating change in diameter with loading. A ring was subsequently machined from the disc and similarly tested to confirm Young's modulu