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Because water is one of the most abundant compounds on earth and was believed to have constant characteristics, early scientists frequently used water as a standard for the measurement of characteristics of other materials. And the standard unit of heat was first based on the amount of heat needed to increase the temperature of water. In the metric system of measurement, the unit of heat was the calorie and was defined as the amount of heat required to increase the temperature of one gram of water by 1° C. The British Thermal Unit (Btu) was the heat unit in the English system, and was defined as the quantity of heat needed to raise the temperature of a pound of water (about a pint) by 1° F.

Unfortunately, water did not prove to be a good standard for heat measurement, for it was soon found that the amount of heat required to raise water temperature changes with the initial temperature of the water. The amount of heat needed to raise water temperature from 40° to 41° for example, is not the same as that needed to raise it from 90° to 91°. Because standards of electrical voltage and of resistance are maintained at national standardizing laboratories throughout the world, scientists agreed internationally to use the heat produced in one second by a current of one ampere through a resistance of one ohm as the standard unit of heat. This heat unit is called the joule, in honor of the early English physicist, James Joule.

Although the joule has been used as the standard unit of heat in most scientific work for a long time, the terms "calorie" and "Btu" have continued to be used in industrial and other practical applications, particularly in countries where the metric system of measurement has not yet been adopted. Because of the continued use of these terms, the calorie has now been arbitrarily defined as equal to 4.1840 joules, which is nearly equivalent to the quantity of heat needed to raise the temperature of a gram of water from 14.5° to 15.5° C. One Btu equals 1055 joules or 252 calories. But by definition and derivation, the calorie and Btu are no longer connected in any way with the properties of water.