Following the development of the concept, it provides an explanation of the trends in contemporary textbooks. The author''s repetition, in his "History and Philosophy of Physics Laboratory", of Joule’s famous experiment – the paddle wheel experiment – with a calorimeter as originally used by Joule and with a calorimeter as proposed in textbooks, is presented, yielding new insight into the phenomenon. Thus, science teachers andstudents will benefit from reading the book as well as historians, philosophers, students of the history and philosophy of science, and all who are interested in knowing about what it is that we call energy.