We are all familiar with the idea of temperature. Mass is a measure of the amount of substance in a system. Velocity is a measure of how fast a system moves. What physical property of the system have you determined if you measure its temperature?

We will begin with the commonsense idea that temperature is a measure of how “hot” or “cold” a system is. As we develop these ideas, we’ll find that temperature $T$ is related to a system’s thermal energy. We defined thermal energy in Chapter 9 as the kinetic and potential energy of the atoms and molecules in a system as they vibrate (a solid) or move around (a gas). A system has more thermal energy when it is “hot” than when it is “cold.” In Chapter 20, we’ll replace these vague notions of hot and cold with a precise relationship between temperature and thermal energy.

To start, we need a means to measure the temperature of a system. This is what a thermometer does. A thermometer can be any small macroscopic system that undergoes a measurable change as it exchanges thermal energy with its surroundings. It is placed in contact with a larger system whose temperature it will measure. In a common glass-tube thermometer, for example, a small volume of mercury or alcohol expands or contracts when placed in contact with a “hot” or “cold” object. The object’s temperature is determined by the length of the column of liquid.

A thermometer needs a temperature scale to be a useful measuring device. In 1742, the Swedish astronomer Anders Celsius sealed mercury into a small capillary tube and observed how it moved up and down the tube as the temperature changed. He selected two temperatures that anyone could reproduce, the freezing and boiling points of pure water, and labeled them 0 and 100. He then marked off the glass tube into one hundred equal intervals between these two reference points. By doing so, he invented the temperature scale that we today call the Celsius scale. The units of the Celsius temperature scale are “degrees Celsius,” which we abbreviate $\text{°}\text{C}$. Note that the degree symbol $\text{°}$ is part of the unit, not part of the number.

The Fahrenheit scale, still widely used in the United States, is related to the Celsius scale by

FIGURE 18.2 shows several temperatures measured on the Celsius and Fahrenheit scales and also on the Kelvin scale.

FIGURE 18.2 Temperatures measured with different scales.