What is the joule apparatus?
A "joule apparatus" is a machine that measured the mechanical equivalence of heat.
James Joule invented a device that can measure the change in temperature of a volume of fluid(heat) per input work (mechanical energy). The main pieces of the apparatus include a thermometer, a chamber for fluid, a propeller submerged in the fluid and a crank for the input work (external).
When the crank is turned, the fluid is churned by the propeller, increasing the average kinetic energy of its particles. Measurements for input work, increase in temperature and volume of fluid can be used to determine a relation between mechanical energy and heat. The amount of energy required to raise 1cc of water by 1 degree Celsius became defined as a Joule.