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Temperature, a measurement of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used. A temperature scale is determined by choosing two reference temperatures and dividing the temperature difference between these two points into a certain number of degrees. The two reference temperatures used for most common scales are the melting point of ice and the boiling point of water. On the Celsius temperature scale, or centigrade scale, the melting point is taken as 0°C and the boiling point as 100°C, and the difference between them is divided into 100 degrees. On the Fahrenheit temperature scale, the melting point is taken as 32°F and the boiling point as 212°F, with the difference between them equal to 180 degrees. The Réaumur scale, used in some parts of Europe, also sets the melting point at zero, but it has an 80-degree temperature difference between 0°R and the boiling point at 80°R. The temperature of a substance does not measure its heat content but rather the average kinetic energy of its molecules resulting from their motions. A one-pound block of iron and a two-pound block of iron at the same temperature do not have the same heat content. Because they are at the same temperature the average kinetic energy of the molecules is the same; however, the two-pound block has more molecules than the one-pound block and thus has greater heat energy. A temperature scale can be defined theoretically for which zero degree corresponds to zero average kinetic energy (see gas laws). Such a point is called absolute zero, and such a scale is known as an absolute temperature scale. The Kelvin temperature scale is an absolute scale having degrees the same size as those of the Celsius temperature scale; the Rankine temperature scale is an absolute scale having degrees the same size as those of the Fahrenheit temperature scale. The relationship between absolute temperature and average molecular kinetic energy is one result of the kinetic-molecular theory of gases.

Temperature

Some of the measurements we take for granted were not known in the ancient world. Among them is temperature. Certainly, ancient people knew that it was cold sometimes and hot sometimes, but no one had a way of putting a number to it. It was Galileo who made the first steps toward a quantitative means of measuring temperature. He noted that gases expand when heated. Using this principle, he developed a thermoscope, an inaccurate gas thermometer. Although Galileo did not know it, the gas he used -- air -- changed in volume according to outside air pressure as well as heating.

A really good thermometer was not made until 1714, when Gabriel Fahrenheit made the first mercury thermometer. Instead of air Fahrenheit used changes in the expansion of mercury to measure temperature. He set 0° as the lowest temperature he could reach by freezing salted water, hoping to avoid negative temperatures.

Anders Celsius did not create a new type of thermometer, but he did create the scale that is most commonly used around the world today. In one of the great mistakes of science, Celsius first set his scale with 0° as the temperature of boiling water and 100° as the temperature of freezing water. Wiser heads prevailed, however, and the scale was reversed. Until 1948 the scale Celsius devised was known in the United States as centigrade, but that year scientists agreed to rename it after its inventor. Gradually the world has gotten used to the idea that 37°C means 37 degrees Celsius, not 37 degrees centigrade.

More accurate thermometers have been developed over the years. Instead of using the simple expansion of materials upon heating, other thermometers use differences in the coefficient of expansion of two metals, the amount of energy emitted at different temperatures by certain substances, or the changes in electrical resistance at different temperatures.

Fahrenheit's idea of a temperature scale with no negative numbers was achieved with the introduction of the Kelvin scale (sometimes called the absolute scale). Lord Kelvin observed in 1848 that Charles's law, which states that gases lose 1/273 of their volume for every degree below 0°C, implies that at -273° the volume becomes zero. He proposed that -273° must be the lowest temperature obtainable, since nothing has a negative volume. This concept has since been verified. Today absolute zero is set more accurately at -273.15°C (-459.7°F). On the Kelvin scale, then, the freezing point of water is 273.15 K (no degree sign is used) and the boiling point is 373.15 K.

General description

The formal properties of temperature are studied in thermodynamics.

Formally, temperature is that property which governs the transfer of thermal energy, or heat, between one system and another. When two systems are at the same temperature, they are in thermal equilibrium and no heat transfer will occur. When a temperature difference does exist, heat will tend to move from the higher temperature system to the lower temperature system, until thermal equilibrium is established. This heat transfer may occur via conduction, convection or radiation.

Temperature is related to the amount of thermal energy or heat in a system. As more heat is added the temperature rises, similarly a decrease in temperature corresponds to a loss of heat from the system. On the microscopic scale this heat corresponds to the random motion of atoms and molecules in the system. Thus, an increase in temperature corresponds in an increase in the rate of movement of the atoms in the system.

Temperature is an intrinsic property of a system, meaning that it does not depend on the system size or the amount of material in the system. Other intrinsic properties include pressure and density. By contrast, mass and volume are extrinsic properties, and depend on the amount of material in the system.

Applications

Temperature plays an important role in almost all fields of science, including physics, chemistry, and biology.

Many physical properties of materials including the phase (solid, liquid, gaseous or plasma), density, solubility, vapor pressure, and electrical conductivity depend on the temperature. Temperature also plays an important role in determining the rate and extent to which chemical reactions occur. This is one reason why the human body has several elaborate mechanisms for maintaining the temperature at 37 °C, since temperatures only a few degrees higher can result in harmful reactions with serious consequences. Temperature also controls the type and quantity of thermal radiation emitted from a surface. One application of this effect is the incandescent light bulb, in which a tungsten filament is electrically heated to a temperature at which significant quantities of visible light are emitted.

Temperature measurement

Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer.

Other important devices for measuring temperature include:

Thermocouples
Thermistors
Resistance Temperature Detector (RTD)
Pyrometers

Units of temperature

The basic unit of temperature (symbol: T) in the International System of Units (SI) is the kelvin (K). One kelvin is formally defined as 1/273.16 of the temperature of the triple point of water (the point at which water, ice and water vapor exist in equilibrium). The temperature 0 K is called absolute zero and corresponds to the point at which the molecules and atoms have the least possible thermal energy. An important unit of temperature in theoretical physics is the Planck temperature (1.4 × 1032 K).

In the field of plasma physics, because of the high temperatures encountered and the electromagnetic nature of the phenomena involved, it is customary to express temperature in electron volts (eV) or kilo electron volts (keV), where 1 eV = 11,605 K.

For everyday applications, it is often convenient to use the Celsius scale, in which 0 °C corresponds to the temperature at which water freezes and 100 °C corresponds to the boiling point of water at sea level. In this scale a temperature difference of 1 degree is the same as a 1 K temperature difference, so the scale is essentially the same as the kelvin scale, but offset by the temperature at which water freezes (273.15 K).

In the United States, the Fahrenheit scale is widely used. On this scale the freezing point of water corresponds to 32 °F and the boiling point to 212 °F.

See temperature conversion formulas for conversions between most temperature scales.

**Comparison of temperature scales**
Comment	kelvin¹	Celsius	Fahrenheit
Absolute zero	0	-273.15	-459.67
Fahrenheit's ice/salt mixture	255.37	-17-78	0
Water freezes at standard pressure	273.15	0	32
Average human body temperature³	310	36.8	98.2
Water boils	373.15	100	212
Titanium melts	1941	1668	3034

Comparison of temperature scales Comment kelvin¹ Celsius Fahrenheit

¹ Only the kelvin, Celsius, Fahrenheit, and Rankine scales are in use today.

² Some numbers in this table have been rounded off.

³ Normal human body temperature is 36.8 °C ±0.7 °C, or 98.2 °F ±1.3 °F.

Fahrenheit to Celsius Converter

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