热动说是一种解释热现象的学说，由伦福德伯爵于1798年引入，并由法国物理学家尼古拉·卡诺进一步发展。这一学说指出热量与机械功在改变内能方面是等价的。这一学说的验证与热功当量的测定密切相关。而在接下来的整个19世纪，随着 鲁道夫·克劳修斯于1850年提出热力学第二定律，这一学说又进一步发展成为较为完备的热力学理论。在1851年，在其著作《论热的动力学理论》（On the Dynamical Theory of Heat）中，开尔文勋爵，基于当时詹姆斯·普雷斯科特·焦耳等人的实验，如是概括了这一学说：“热并非一种物质，而是一种运动现象，我们认为在机械功和热量之间必存在着一如因与果一般的等量关系。”

In the history of science, the theory of heat or mechanical theory of heat was a theory, introduced in 1798 by Sir Benjamin Thompson (better known as 'Count Rumford'), and developed more thoroughly in 1824 by the French physicist Sadi Carnot, that heat and mechanical work are equivalent. It is related to the mechanical equivalent of heat. Over the next century, with the introduction of the second law of thermodynamics in 1850 by Rudolf Clausius, this theory evolved into the science of thermodynamics. In 1851, in his "On the Dynamical Theory of Heat", William Thomson outlined the view, as based on recent experiments by those such as James Joule, that “heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect.”