# The Encyclopedia Americana (1920)/Modulus

**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.