The Joule paddle wheel

A good example of the quantitative measurement of stirring work is the set of experiments conducted by James Joule in the 1840s to determine the mechanical equivalent of heat. In effect, he determined the quantity of dissipative stirring work that could replace the heat needed for the same temperature increase. 

Joule s apparatus contained the paddle wheel shown in Fig. 3.12. It consisted of eight sets of metal paddle arms attached to a shaft in a water-fiUed copper vessel. When the shaft rotated, the arms moved through openings in four sets of stationary metal vanes fixed inside the vessel, and churned the water. The vanes prevented the water from simply moving around in a circle. The result was turbulent motion (shearing or viscous flow) in the water and an increase in the temperature of the entire assembly. 

A typical experiment performed by Joule is described in Prob. 3.10 on page 99. His results for the mechanical equivalent of heat, based on 40 such experiments at average temperatures in the range 13 °C-16 °C and expressed as the work needed to increase the temperature of one gram of water by one kelvin, was 4.165J. This value is close to the modern value of 4.1855 J for the 15 °C calorie, the energy needed to raise the temperature 

Iftermodynam/cs ancf Ctem/sfry, second edition, version 3 2011 by Howard DeVoe. Latest version . chem.umd.edu/thermobook 

James Joule drove the final nails into the coffin of the caloric theory by his experimental demonstrations of the mechanical equivalent of heat. 

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