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How did Joule design his experiments and why did he use the apparatus he did?

Joule wanted to cause mechanical energy to disappear in as many ways as possible and to measure the amount of heat developed as a result.

The page from Joule's notebook shows his drawing of the apparatus for this experiment. If the weights descended slowly and uniformly, this work would all be expended in overcoming the resistance of the water to the motion of the paddlewheels through it.

Students at the University of Oldenburg, in Germany, recreated the experiment. They used the apparatus Joule described in his paper but did not get the same results. They produced a different version of the equipment - shown above - which did work. They also found it was necessary to have two people to carry out the experiment: one to wind up the weights and one to take the thermometer readings.

This shows the value of recreating experiments rather than relying on reported descriptions.