This website would like to remind you: Your browser (Apple Safari 4) is out of date. Update your browser for more × security, comfort and the best experience on this site. Encyclopedic Entry thermometer For the complete encyclopedic entry with media resources, visit: http://education.nationalgeographic.com/encyclopedia/thermometer/ 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 1 of 5 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. Other Thermometers Today, specialized thermometers are used for a variety of purposes. A cryometer measures very low temperatures, for instance. Cryometers are used to measure temperatures in space. Pyrometers are used to measure very high temperatures. The steel industry uses pyrometers to measure the temperatures of iron and other metals. Astronomers use infrared thermometers to measure temperatures in space, for instance. Infrared thermometers detect infrared radiation at great distances and correlate it to a specific surface temperature. In 1965, an infrared thermometer detected radiation with a temperature of 3 kelvins (-270 degrees Celsius/-454 degrees Fahrenheit) in all directions in space. Astronomers deduced that this very cold radiation was probably the faint remnant of the Big Bang—the expansion of the universe from a single point that began approximately 13.82 billion years ago. Athletic trainers use pill thermometers in order to prevent and treat heat-related illnesses like heatstroke. After being swallowed, a pill thermometer transmits information about the body's core temperature for 18 to 30 hours. Pill thermometers use liquid crystals to track changes in body heat and transmit radio waves to a source outside the body, which records and displays this data. Researchers at Harvard University have developed a nanothermometer that is able to measure temperature variations inside a single living cell. Using a nanowire “needle,” researchers inject carbon nanocrystals into a cell’s interior. These crystals are less than 5 nanometers in length (a sheet of paper is 100,000 nanometers thick) and detect incredibly small fluctuations in temperature. Scientists are now developing nanocrystal technologies that can change cellular temperatures. These technologies may ultimately be used in medical treatments that overheat and kill cancer at the cellular level. VOCABULARY Term Part of Speech Definition absolute zero noun hypothetical coldest possible temperature where all molecular motion stops (- 273.16 degrees Celsius and -459.69 degrees Fahrenheit). Also called zero Kelvin. air pressure noun force pressed on an object by air or atmosphere. astronomer noun person who studies space and the universe beyond Earth's atmosphere. Big Bang noun (12-20 billion years ago) theoretical event where a small, dense, hot body of matter exploded, creating the expanding universe. body temperature noun heat energy radiated by a person or other animal. Also called normothermia or euthermia. For humans, resting body temperature is 37 degrees Celsius or 98.6 degrees Fahrenheit. calculate verb to reach a conclusion by mathematical or logical methods. cell noun smallest working part of a living organism. Celsius scale noun scale for measuring surface temperature, used by most of the world, in which the boiling point of water is 100 degrees. contract verb to shrink or get smaller. convert verb to change from one thing to another. 2 of 5 correlate verb to bring different sets of data into order, or establish a relationship or connection between them. cryometer noun thermometer for measuring very low temperatures. data plural noun (singular: datum) information collected during a scientific study. deduce verb to reach a conclusion based on clues or evidence. detect verb to notice. device noun tool or piece of machinery. display verb to show or reveal. electric current noun flow of electricity, or charged particles, through a conductor. electronic noun device for measuring temperature electronically. thermometer energy noun capacity to do work. environment noun conditions that surround and influence an organism or community. essential adjective needed. ethanol noun type of grain alcohol used as biofuel. expand verb to grow. Fahrenheit scale noun scale for measuring surface temperature used by Belize, Liberia, Myanmar, and the United States. faint adjective weak or barely detectable. familiar adjective well-known. fluctuation noun change, or motion from one point to another. food noun material, usually of plant or animal origin, that living organisms use to obtain nutrients. freezing adjective at or below 0 degrees Celsius (32 degrees Fahrenheit). gas noun state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion. heat noun energy that causes a rise in temperature. industry noun activity that produces goods and services. infrared radiation noun part of the electromagnetic spectrum with wavelengths longer than visible light but shorter than microwaves. infrared noun device for measuring temperature using infrared radiation. thermometer inject verb to force something (usually a liquid) into a cavity or tissue. instrument noun tool. Kelvin scale noun scale for measuring temperature where zero Kelvin is absolute zero, the absence of all energy. kerosene noun flammable liquid used as fuel. 3 of 5 liquid noun state of matter with no fixed shape and molecules that remain loosely bound with each other. liquid crystal noun substances that have liquid qualities, but whose molecules are arranged like a crystal. Liquid crystals are not a liquid form of a solid crystal. mass noun unit of measurement (abbreviated m) determined by an object's resistance to change in the speed or direction of motion. maximum noun thermometer designed to display the highest temperature recorded between two thermometer settings. measurement noun process of determining length, width, mass (weight), volume, distance or some other quality or size. medical adjective having to do with the study of medicine or healing. mercury noun chemical element with the symbol