Temperature

There is no easy way to measure directly the average molecular translational energies in an object. Therefore, temperature is determined indirectly by measuring a temperature-dependent property of a device that is in thermal equilibrium with the object to be measured. We call such a device a thermometer. One of the earliest kinds of thermometers, still in use today, has a liquid in a glass bulb attached to a glass capillary tube—a tube with a very narrow bore. The tube is sealed to make the thermometer independent of atmospheric pressure. The expansion and contraction of the liquid volume as the temperature goes up and down results in a rise and fall of the thin column of liquid in the tube, which can be calibrated in terms of temperature.

Early thermometry used water or water-alcohol mixtures as the liquid. Water turned out to be an especially poor choice, however, because the volume of water does not change uniformly with changes in temperature. When cooled down from room temperature, liquid water at first contracts. Then, as the water approaches its freezing point, it expands. This unusual property of expansion upon cooling means that a water thermometer not only goes down as the temperature falls, but it sometimes goes up. This is certainly not a desirable characteristic of a thermometer liquid.