Cavendish Laboratory, Madingley Road,
Cambridge, CB3 OHE, UK.
We present a calculation of the rubber elasticity (with finite chain extensibility) of an isotropic crosslinked network (of freely jointed chains) under affine deformation. The result is derived in its full tensorial notation, and its predictions for general biaxial deformations are presented. The negative value of , observed experimentally, is explained.