Joule recognized the usefulness to him of making small things visible. So he commissioned Dancer to design a thermometer, which with the aid of a lens, allowed minute movements in a thin column of mercury to be magnified and measured accurately. Dancer’s thermometers, Joule claimed, enabled him to measure temperatures to an accuracy of better than one-tenth of a degree Fahrenheit. When used with the paddle-wheel apparatus, their readings supported the conclusions from Joule’s dynamo experiments.
