A thermometer is an instrument that measures temperature. It can measure the temperature of a solid such as food, a liquid such as water, or a gas such as air. The three most common units of measurement for temperature are Celsius, Fahrenheit, and kelvin.
The Celsius scale is part of the metric system. The metric system of measurement also includes units of mass, such as kilograms, and units of length, such as kilometers. The metric system, including Celsius, is the official system of measurement for almost all countries in the world. Most scientific fields measure temperature using the Celsius scale. Zero degrees Celsius is the freezing point of water, and 100 degrees Celsius is the boiling point of water.
Three nations do not use the Celsius scale. The United States, Burma, and Liberia use the Fahrenheit scale to measure temperature. However, even in these countries, scientists use the Celsius or kelvin scale to measure temperature. Water freezes at 32 degrees Fahrenheit and boils at 212 degrees Fahrenheit.
The Kelvin scale is used by physicists and other scientists who need to record very precise temperatures. The kelvin scale is the only unit of measurement to include the temperature for "absolute zero," the total absence of any heat energy. This makes the kelvin scale essential to scientists who calculate the temperature of objects in the cold reaches of outer space. Water freezes at 273 kelvins, and boils at 373 kelvins. We do not read outdoor temperatures in the kelvin scale because it uses such large numbers—a 75-degree Fahrenheit day would be read as 297 kelvins!
Types of Thermometers
Liquid Thermometers
Liquid expands at a regular, measureable rate when it is heated. For this reason, a common form of thermometer contains a liquid in a narrow glass tube. Mercury is one of the most familiar materials used in liquid thermometers. Other liquids, such as kerosene or ethanol, may also be used in these types of thermometers.
When heat rises, the liquid expands from a bowl or bulb into the empty area, climbing up the tube. When the temperature falls, the liquid contracts and goes back down. Liquid thermometers often include both Celsius and Fahrenheit temperature scales, which are displayed on either side of the tube.
A maximum thermometer is a familiar type of liquid thermometer. In a maximum thermometer, the liquid is pushed up the glass tube, but cannot fall easily when the temperature lowers. The maximum temperature over a set period of time can be observed after the thermometer is removed from the environment. Maximum thermometers are commonly used to measure a person’s body temperature.
Liquid thermometers can be limited by the type of liquid used. Mercury, for instance, becomes a solid at -38.83 degrees Celsius (-37.89 degrees Fahrenheit). Mercury thermometers cannot measure temperatures below this point. Alcohols, such as ethanol, boil at about 78 degrees Celsius (172 degrees Fahrenheit). They cannot be used to measure temperatures above this point.
Electronic Thermometers
Mercury and other liquid thermometers cannot be used to measure temperatures in kelvins. Kelvin thermometers are usually electric devices that can record tiny variations in radiation. These variations would not be visible and may not change air pressure enough to raise the level of mercury in a liquid thermometer.
Electronic thermometers work with an instrument called a thermistor. A thermistor changes its resistance to an electric current based on the temperature. A computer measures the thermistor’s resistance and converts it to a temperature reading.