The original Joule experiment consists of a receptacle filled with water and a mechanism with spinning plates. The kinetic energy of the plates is transformed into heat, because the force of gravity performs work on the weight falling a distance . This gave an experimental confirmation of the equivalence between heat and work, now defined to be exactly 1 calorie for every 4.184 joules and called a "thermochemical calorie".
Wolfram Demonstrations Project
Published: March 7 2011