Temperature is a measure of how fast the molecules in a substance are moving. When heat energy is added to an object, its molecules move faster and its temperature increases. A slice of apple pie at 40°C contains more heat energy than a same-sized slice of the same pie at 30°C. Temperatures can be measured accurately using devices called thermometers. Older thermometers used mercury, the only metal that is liquid at room temperature. Because mercury is poisonous, many types of thermometer nowadays usually use some type of liquid alcohol.

Thermometers

When a substance gets hotter, it expands to a greater volume. Nearly all substances have this property, which is called thermal expansion. It is the basis of how thermometers work. In a thermometer, liquid is enclosed in a tall, narrow glass (or plastic) tube. When the temperature increases, the liquid increases in volume, and the height of the column of liquid in the tube rises. The height is proportional to the increase in temperature. Marks, called calibrations, alongside the tube, give a measure of temperature according to a certain scale.

The first thermometers

A thermoscope is a simple device in which a liquid, such as water, rises and falls as the temperature changes. Usually, the water is held in a tube that leads up to a container filled with air. When the device cools, the air contracts and the water rises up the tube. The higher the water, the higher the temperature.

​​​​​​​The first thermoscope was built by Galileo Galilei around 1592. In later versions, made in the 17th century, calibrations were added, producing the first thermometers. But each one was unique: there was no standard scale.

Mercury thermometer

Gabriel Daniel Fahrenheit (1668–1736), a German physicist, glass-blower and instrument-maker working in Holland, decided to improve the design of the thermometer. Thermometers of the time depended on the expansion and contraction of alcohol within a glass tube, but truly accurate ones were difficult to build.

Fahrenheit scale

To calibrate (mark a scale of measurement on) his new thermometer, Fahrenheit needed to choose two temperatures that were fixed in nature, mark them on the thermometer, then divide the length of tube into a number of equal degrees.

Instead of using the freezing point of water for the bottom of the scale—as suggested by Isaac Newton in 1701—Fahrenheit, accustomed to Holland's freezing winters, decided to adopt the freezing point of salt water for the lower extreme. At the top end, he chose the temperature of blood in the human body. Unfortunately, neither of these temperatures are fixed points, which limited the accuracy of his system.

Celsius and Kelvin

In 1742, the Swedish astronomer Anders Celsius (1701–44) proposed a centigrade scale, which took the freezing and boiling temperatures of pure water at normal pressure as 100° and 0°. Because of its simplicity, it was quickly adopted for general use worldwide, apart from in English-speaking countries. However, it was soon reversed so that 100° became the boiling temperature and 0° the freezing temperature. In the USA, the Fahrenheit system is (outside scientific circles) still used today.

The SI unit of measurement of temperature is the kelvin (after British physicist William Thomson, Baron Kelvin, 1824–1907). Its lowest point is absolute zero, the coldest possible temperature, –273.15° on the Celsius scale.