Alexis Thérèse Petit (2.10.1791-21.06.1820) — French physicist. Born in Vesoul. Graduated from the École Polytechnique (1809). In 1810 he became a professor at a lycée (Paris), and from 1815 was a professor at the École Polytechnique.
His work concerned heat and molecular physics. Together with P. Dulong he studied the cooling of bodies in a vacuum and derived (1818) a formula for the rate of cooling of heated bodies. He developed a method of studying the thermal conductivity of gases, and also methods of determining the thermal expansion and specific heat capacity of solids. In 1819 he established empirically, with Dulong, that the atomic heat capacity of all simple solids at constant volume is almost invariable and equal to 6 cal/(mol·K) (the Dulong–Petit law), and he invented (1816) the cathetometer.