Young's Modulus Determined with Small Stresses

An interferometer method similar to that used by E. Gr\"uneisen was used to determine the extensions of metallic rods of brass, steel, copper, aluminum and nickel under small stresses. The largest stresses applied were considerably less than those usually used in commercial testing. For the substances examined Young's Modulus was found to increase very rapidly as the stress increased from zero, reach a maximum at a comparatively small stress, and then to decrease almost exponentially to the ordinary value. The annealing of the rod may be connected with this divergence from Hooke's law with small stresses, as slight differences were found on reannealing some of the rods used.