James Prescott Joule was the first experimentalist who accurately measured the mechanical energy equivalent to some amount of heat energy, the mechanical equivalent of heat. This measurement determines the amount of mechanical energy expressed in Joule (enr or ft.lb) which is equivalent to heat energy of 1 cal.
Mechanical energy may be used to do work and work produces heat. On the other hand, heat may be converted to work or mechanical energy. It indicates that there is a natural relationship between work and heat. This relation is known as Joule’s law.
Joule’s Law:
If some amount of work is entirely converted into heat, then the work done and the heat produced are proportional to each other.
If W = work done and H = heat produced, then from Joule’s law,
W ∝ H
or, W = JH
Here, J is a constant. This constant is called the mechanical equivalent of heat or Joule’s equivalent.