The reason we are interested in thermal expansion is that it is the physical mechanism that makes possible the most common kind of thermometer. The size or volume of a material changes slightly with temperature, and this can give visible effects that can be used to determine the temperature.

Most materials expand when their temperature goes up, most of the time. The best known exception is liquid water in the temperature interval 0^o C --4^o C (just above the melting temperature), where it is contracting with increasing temperature. The change in size caused by thermal expansion is never very large: near room temperature, gases increase in volume by less than 0.5% per degree Celsius, and the rate of thermal expansion of liquids and solids is 100 times smaller. For example, a steel post that is exactly 1 meter long at 0^o C is 1.2 millimeters longer at 100^o C. This seems negligible until we discover that it would take a 15 ton force to keep the post from expanding! (this assumes the post is a rod 1 inch in diameter). So it is very important to design bridges and buildings and car engines so that thermal expansion can take place.

When the temperature change is uniform throughout the object, the change in dimensions is also uniform: all distances get larger by the same fractional amount. So a washer gets larger and the hole in the washer also gets larger. However, different materials have different thermal expansion, so that a steel washer that exactly fits on a brass rod at one temperature will be too big or too small at another.

Here are some more examples:

• The traditional way to fasten an iron rim to a wooden wheel (or put an iron hoop around a wooden barrel) was to heat the rim red hot. This made it slightly larger, so that the wheel could be forced into the rim; after the rim cooled and shrank, it was firmly held in place. The modern version of this is that one should not mix bolts and nuts made of different metals -- there will be a temperature range in which the bolt is larger than the nut (and the nut is then frozen in place), and another range where the nut is bigger than the bolt (and then it is not held in place securely).
• If the ocean becomes 1 degree (^oF) warmer, its volume will increase by 0.01%. Since the ocean is several miles deep, this implies that the surface will rise about a foot, giving a change in the sea level. In the process, the beach line moves landwards 20 feet. People who live close to the ocean find this alarming.

Many kinds of thermometers

The thermometers in the kit make use of thermal expansion to determine the temperature. The red liquid (which is colored alcohol) expands much more than the glass tube that holds it, so that the part of the tube that is filled changes. The thermal sensor also depends on thermal expansion, because the tape and the aluminum foil expand different amounts, causing a curvature. Because thermal expansion is simple and reproducible, it is used in many kinds of thermometers.

However, there are other kinds of thermometers. The temperature of a steel furnace or a glass kiln is determined by the color of the light that emitted. The electrical properties of some circuit elements are sensitive to temperature, and we can expect that electronic thermometers will become common in a few years, because they are very easy to make. We will meet later a kind of thermometer, that tells the temperature by changing color.

Even though these different kinds of thermometers are based on different physical mechanisms, they all measure the same physical property, and will all give the same value if they have been calibrated properly.

Melting and freezing, and boiling

At a change in phase -- when a liquid turns into gas, or solid turns into liquid -- we can get large changes in volume. Although it is "thermal" and it is "expansion", it is not the thermal expansion effect that we have been studying. It differs in these ways:

• It involves a complete rearrangement of the structure of the material.
• The change in density is not uniform and gradual throughout the material. As a block of ice melts, some parts are solid while others are liquid.
• A change of phase in a pure substance (like water) takes place over a very small temperature range, whereas thermal expansion takes place in almost every temperature range. We couldn't make a thermometer based on the change of density of water as it freezes (but we could use the freezing temperature to calibrate a thermometer).

Atoms