Joule performed a variety of experiments in which he converted a carefully measured quantity of work, through friction, into an equally measured quantity of heat. For example, in one experiment Joule used falling masses to propel a paddle wheel in a thermally insulated, water-filled container. Measurements of the distance through which the masses fell and the temperature change of the water allowed Joule to determine the work performed and the heat produced. With many such experiments, Joule demonstrated that the ratio between work performed and heat produced was constant. In modern units, Joule's results are stated by the expression: