MODULUS, in mathematics, a constant referring to properties of matter in certain equations. As stress is proportional to strain within the elastic limits, some constant quantity may be introduced making this proportionality into an equality. In dealing with strength of materials, such a constant is called a modulus. Thus in Hooke's law, which says that extension, as of a bar, is proportional to the extending force, the constant which converts this proportionality into an equality is called the modulus of elasticity, or Young's modulus, and is denoted by E. Where the elastic limits are not exceeded, the transverse strain — i.e., the contraction per unit of transverse dimension — is from one-third to one-fourth the longitudinal strain. The symbol denoting the modulus of elasticity of bulk is K. It denotes the lessening of bulk per unit cube, usually per cubic inch, under hydrostatic stress. See Elasticity.