Mechanical energy can be converted into heat, and heat can be converted into mechanical energy. This relationship is known as the mechanical equivalent to heat and it is an important concept fundamental to thermodynamics which applies ideas related to heat and work to create useful systems (i.e. power plants, engines, refrigerators ...). The mechanical equivalent to heat phenomena was first tested by James Joule in 1843. In his experiment, he used the change in potential energy of a falling mass to stir water. As the water was stirred, the temperature of the water increased just like it was placed over a flame. This proved that the work done on a system (the falling mass) can be equated to an increase in water temperature (heat energy)