In deformation by Power law, which is dependent upon not only the strain rate but a numerical value A dependent upon Coble creep and grain boundary diffusion and other factors, the value of shear stress changes. If the value of the power n of sigma equals unity, then the material is linearly viscous but if n exceeds 1 then the tangent to the stress strain curve gives the value of effective viscosity. The tangent is to curve and this straight line gives not the mean but the effective viscosity of the material.