Abstract
A constant stress extension apparatus designed to measure extension-time curves for plastics for which Poisson's ratio may be taken as 0.5 is described. Extension up to 50% can be measured, and the provision of low- and high-magnification optical lever systems for measurement of the extension enables small and large movements to be measured. The constant stress is obtained by means of a cam, the shape of which has been computed. Some preliminary results are given of the behaviour of a nitrocellulose plastic, in order to illustrate the use of the instrument. This material does not obey the `Boltzmann Superposition Principle' for large stresses.