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IF the temperature of a linear viscoelastic solid creeping under a constant stress is changed abruptly from T0 to T then the ratio of the instantaneous creep rates at the time of the T-jump is illustration in which illustration β is a parameter which depends on the viscoelastic properties of the solid1, κ a parameter which accounts for the perturbation due to thermoelastic stresses generated by the T-jump2 and ΔH the activation energy for creep. If β=κ=1, equation (1) reduces to the Dorn equation3, illustration so that ΔH may be determined from a measurement of eT0 and eT. Here we show how ΔH may be determined when κ=1 but β takes values greater or less than unity depending on the direction of the T-jump. Open image in new window Open image in new window Open image in new window