A mercury thermometer was used to measure the changing temperature of the water. This experiment was repeated with varying lengths and thicknesses of copper and iron wires and so the galvanometer needle deflected to various degrees depending on this. The results of this experiment were noted in his 1841 publication. In it, he states his concluding remarks, “We see, therefore, that when a current of voltaic electricity is propagated along a metallic conductor, the heat evolved in a given time is proportional to the resistance of the conductor multiplied by the square of the electric intensity.”
DISCOVERER
Here is a portrait[3] of James Prescott Joule: