STRENGTH OF MATERIALS). The longitudinal strain e is always
accom panied by a strain e' of the transverse dimen sions; for a
given material e'=cre where a is constant for isotropy. Thus if
a rod receives a longitudinal strain e the strain of any
straight line in a cross-section is ae: at is not an areal
strain. In engineering. m is used for a. a is called Poisson's
ratio after Poisson, who in 1828 calculated it to be for all
materials; experiment has not verified his prediction. (2) For
shear stress q and strain q-=-14; p is the modulus of shear or
of rigidity. Young was the first to point out that resistance to
(detrusion,p as he called shear, was different from resistance
to stretching. But he did not introduce the shear modulus; this
was done by Navier in 1833. The symbol p was first used by Lame
in 1852; in books on the strength of materials N and G are used
for p. .(3) A constant normal stress p over the entire surface
of a body produces a volumetric strain A, called the dilatation,
where k being the bul