Page:Elementary Text-book of Physics (Anthony, 1897).djvu/213

§ 171] plates. He gave the preference to the smallest value, and it has been generally accepted as the mechanical equivalent. This mechanical equivalent is called Joule's equivalent, and is represented by $$J.$$ In absolute units, according to the later determinations of Griffiths, it is about 41,982,000,000 ergs per calorie.

Rowland has repeated Joule's experiment with water; but he caused the paddle-wheel to revolve by means of an engine, and determined the moment of the couple required to prevent the revolution of the calorimeter. Fig. 68 shows the apparatus. The shaft of the paddle-wheel projects through the bottom of the calorimeter, and is driven by means of a bevel-gear. The vessel $$A$$ is suspended from $$C$$ by a torsion wire, and its tendency to rotate balanced by weights attached to cords which act upon the circumference of a pulley $$D.$$ By this disposition of the apparatus he was able to expend about one half a horse-power in the calorimeter, and obtain a