For many years, an equation to express the relationship of stress to strain has been desired, and recently a study of the change of properties resulting from the cold working of a metal has assumed important proportions. For stresses below the elastic limit, Hooke's law and Poisson's ratio are applicable, but above the elastic limit relation to stress to strain follows a different law. Copper and steel are both ductile materials, and it is, therefore, logical to assume that they will show similarity of characteristics. Copper and steel have individual peculiarities, but copper appears to be erratic in its behavior. For example, the modulus of elasticity of steel varies, according to some law resembling a hyperbolic function of the cold working. However, copper does not follow a similar law. The investigation of cold worked copper is of importance in practice. In the manufacture of copper wire, for instance, it is desirable to know how the elastic limit, the maximum load, and the plastic working limit vary as the degree of cold drawing increases. / M.S.