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.
 
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.
thermometer
Mercury thermometers were used by the inventors of both the Celsius (Anders Celsius) and Fahrenheit (Daniel Gabriel Fahrenheit) temperature scales.
World’s Largest
Located outside of Las Vegas, the world’s largest thermometer measures 134 feet high and commemorates the highest temperature ever recorded in North America: 134 degrees Fahrenheit. This temperature was measured in nearby Death Valley in 1934.

Fahrenheit
Daniel Gabriel Fahrenheit was a Polish physicist who invented one of the most familiar types of thermometers, which uses mercury in glass. Fahrenheit also had a temperature scale named after him.

Degrees of Temperature
The Celsius and Fahrenheit scales use degrees to measure temperature. For instance, water boils at 100 degrees Celsius and 212 degrees Fahrenheit.

The Kelvin scale does not use degrees. It uses the kelvin, abbreviated K, as a unit of measurement. Temperatures in kelvins are never read as degrees kelvin or kelvin degrees. Water boils at 373 kelvins.

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.

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 thermometer
Noun

device for measuring temperature electronically.

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.

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 thermometer
Noun

device for measuring temperature using infrared radiation.

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.

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 thermometer
Noun

thermometer designed to display the highest temperature recorded between two 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 Hg.

metal
Noun

category of elements that are usually solid and shiny at room temperature.

metric system
Noun

series of standard weights and measurements used by most countries (except the United States, Liberia, and Burma) and throughout the scientific world. Also called the International System of Units or SI.

nanocrystal
Noun
particle having at least one dimension smaller than 100 nanometres (nm) and composed of atoms in a crystalline arrangement. Semiconductor nanocrystals having dimensions smaller than 10nm are called quantum dots.
nanothermometer
Noun

thermometer that measures temperatures in objects smaller than a micrometer.

observe
Verb

to watch.

physicist
Noun

person who studies the relationship between matter, energy, motion, and force.

pill thermometer
Noun

device for measuring temperature that can be ingested.

precise
Adjective

exact.

prevent
Verb

to keep something from happening.

pyrometer
Noun

thermometer for measuring very high temperatures.

radiation
Noun

energy, emitted as waves or particles, radiating outward from a source.

radio wave
Noun

electromagnetic wave with a wavelength between 1 millimeter and 30,000 meters, or a frequency between 10 kilohertz and 300,000 megahertz.

remnant
Noun

something that is left over.

specialize
Verb

to study, work, or take an interest in one area of a larger field of ideas.

steel
Noun

metal made of the elements iron and carbon.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

thermistor
Noun

electrical-resistance device whose resistance fluctuates with temperature.

Noun

device that measures temperature.

transmit
Verb

to pass along information or communicate.

variation
Noun

difference.

visible
Adjective

able to be seen.