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Temperature is one of those aspects of the everyday world that seems rather abstract when viewed from the standpoint of physics. In scientific terms, it is not simply a measure of hot and cold, but an indicator of molecular motion and energy flow. Thermometers measure temperature by a number of means, including the expansion that takes place in a medium such as mercury or alcohol. These measuring devices are gauged in several different ways, with scales based on the freezing and boiling points of wateras well as, in the case of the absolute temperature scale, the point at which all molecular motion virtually ceases.



Energy appears in many forms, including thermal energy, or the energy associated with heat. Heat is internal thermal energy that flows from one body of matter to anotheror, more specifically, from a system at a higher temperature to one at a lower temperature.

Two systems at the same temperature are said to be in a state of thermal equilibrium. When this occurs, there is no exchange of heat. Though people ordinarily speak of "heat" as an expression of relative warmth or coldness, in physical terms, heat only exists in transfer between two systems. It is never something inherently part of a system; thus, unless there is a transfer of internal energy, there is no heat, scientifically speaking.


Thus, heat cannot be said to exist unless there is one system in contact with another system of differing temperature. This can be illustrated by way of the old philosophical question: "If a tree falls in the woods when there is no one to hear it, does it make a sound?" From a physicist's point of view, of course, sound waves are emitted whether or not there is an ear to receive their vibrations; but, consider this same scenario in terms of heat. First, replace the falling tree with a hypothetical object possessing a certain amount of internal energy; then replace sound waves with heat. In this case, if this object is not in contact with something else that has a different temperature, it "does not make a sound"in other words, it transfers no internal energy, and, thus, there is no heat from the standpoint of physics.

This could even be true of two incredibly "hot" objects placed next to one another inside a vacuuman area devoid of matter, including air. If both have the same temperature, there is no heat, only two objects with high levels of internal energy. Note that a vacuum was specified: assuming there was air around them, and that the air was of a lower temperature, both objects would then be transferring heat to the air.


If heat is internal thermal energy in transfer, from whence does this energy originate? From the movement of molecules. Every type of matter is composed of molecules, and those molecules are in motion relative to one another. The greater the amount of relative motion between molecules, the greater the kinetic energy, or the energy of movement, which is manifested as thermal energy. Thus, "heat"to use the everyday term for what physicists describe as thermal energyis really nothing more than the result of relative molecular motion. Thus, thermal energy is sometimes identified as molecular translational energy.

Note that the molecules are in relative motion, meaning that if one were "standing" on a molecule, one would see the other molecules moving. This is not the same as movement on the part of a large object composed of molecules; in this case, molecules themselves are not directly involved in relative motion.

Put another way, the movement of Earth through space is an entirely different type of movement from the relative motion of objects on Earthpeople, animals, natural forms such as clouds, manmade forms of transportation, and so forth. In this example, Earth is analogous to a "large" item of matter, such as a baseball, a stream of water, or a cloud of gas.

The smaller objects on Earth are analogous to molecules, and, in both cases, the motion of the larger object has little direct impact on the motion of smaller objects. Hence, as discussed in the Frame of Reference essay, it is impossible to perceive with one's senses the fact that Earth is actually hurling through space at incredible speeds.


The relative motion of molecules determines phase of matterthat is, whether something is a solid, liquid, or gas. When molecules move quickly in relation to one another, they exert a small electromagnetic attraction toward one another, and the larger material of which they are a part is called a gas. A liquid, on the other hand, is a type of matter in which molecules move at moderate speeds in relation to one another, and therefore exert a moderate intermolecular attraction.

The kinetic theory of gases relates molecular motion to energy in gaseous substances. It does not work as well in relation to liquids and solids; nonetheless, it is safe to say thatgenerally speakinga gas has more energy than a liquid, and a liquid more energy than a solid. In a solid, the molecules undergo very little relative motion: instead of bumping into each other, like gas molecules and (to a lesser extent) liquid molecules, solid molecules merely vibrate in place.

Understanding Temperature

As with heat, temperature requires a scientific definition quite different from its common meaning. Temperature may be defined as a measure of the average molecular translational energy in a systemthat is, in any material body.

Because it is an average, the mass or other characteristics of the body do not matter. A large quantity of one substance, because it has more molecules, possesses more thermal energy than a smaller quantity of that same substance. Since it has more thermal energy, it transfers more heat to any body or system with which it is in contact. Yet, assuming that the substance is exactly the same, the temperature, as a measure of average energy, will be the same as well.

Temperature determines the direction of internal energy flow between two systems when heat is being transferred. This can be illustrated through an experience familiar to everyone: having one's temperature taken with a thermometer. If one has a fever, one's mouth will be warmer than the thermometer, and therefore heat will be transferred to the thermometer from the mouth until the two objects have the same temperature. At that point of thermal equilibrium, a temperature reading can be taken from the thermometer.


The principles of thermodynamicsthe study of the relationships between heat, work, and energy, show that heat always flows from an area of higher temperature to an area of lower temperature. The opposite simply cannot happen, because coldness, though it is very real in terms of sensory experience, is not an independent phenomenon. There is not, strictly speaking, such a thing as "cold"only the absence of heat, which produces the sensation of coldness.

One might pour a kettle of boiling water into a cold bathtub to heat it up; or put an ice cube in a hot cup of coffee "to cool it down." These seem like two very different events, but from the standpoint of thermodynamics, they are exactly the same. In both cases, a body of high temperature is placed in contact with a body of low temperature, and in both cases, heat passes from the high-temperature body to the low-temperature one.

The boiling water warms the tub of cool water, and due to the high ratio of cool water to boiling water in the bathtub, the boiling water expends all its energy raising the temperature in the bathtub as a whole. The greater the ratio of very hot water to cool water, on the other hand, the warmer the bathtub will be in the end. But even after the bath water is heated, it will continue to lose heat, assuming the air in the room is not warmer than the water in the tub. If the water in the tub is warmer than the air, it immediately begins transferring thermal energy to the low-temperature air until their temperatures are equalized.

As for the coffee and the ice cube, what happens is quite different from, indeed, opposite to, the common understanding of the process. In other words, the ice does not "cool down" the coffee: the coffee warms up the ice and presumably melts it. Once again, however, it expends at least some of its thermal energy in doing so, and as a result, the coffee becomes cooler than it was.

If the coffee is placed inside a freezer, there is a large temperature difference between it and the surrounding environmentso much so that if it is left for hours, the once-hot coffee will freeze. But again, the freezer does not cool down the coffee; the molecules in the coffee respond to the temperature difference by working to warm up the freezer. In this case, they have to "work overtime," and since the freezer has a constant supply of electrical energy, the heated molecules of the coffee continue to expend themselves in a futile effort to warm the freezer. Eventually, the coffee loses so much energy that it is frozen solid; meanwhile, the heat from the coffee has been transferred outside the freezer to the atmosphere in the surrounding room.


Temperature is related to the concept of thermal equilibrium, and has an effect on thermal expansion. As discussed below, as well as within the context of thermal expansion, a thermometer provides a gauge of temperature by measuring the level of thermal expansion experienced by a material (for example, mercury) within the thermometer.

In the examples used earlierthe thermometer in the mouth, the hot water in the cool bathtub, and the ice cube in the cup of coffeethe systems in question eventually reach thermal equilibrium. This is rather like averaging their temperatures, though, in fact, the equation involved is more complicated than a simple arithmetic average.

In the case of an ordinary mercury thermometer, the need to achieve thermal equilibrium explains why one cannot get an instantaneous temperature reading: first, the mouth transfers heat to the thermometer, and once both mouth and thermometer reach the same temperature, they are in thermal equilibrium. At that point, it is possible to gauge the temperature of the mouth by reading the thermometer.


Development of the Thermometer

A thermometer can be defined scientifically as a device that gauges temperature by measuring a temperature-dependent property, such as the expansion of a liquid in a sealed tube. As with many aspects of scientific or technological knowledge, the idea of the thermometer appeared in ancient times, but was never developed. Again, like so many other intellectual phenomena, it lay dormant during the medieval period, only to be resurrected at the beginning of the modern era.

The Greco-Roman physician Galen (c. 129-216) was among the first thinkers to envision a scale for measuring temperature. Of course, what he conceived of as "temperature" was closer to the everyday meaning of that term, not its more precise scientific definition: the ideas of molecular motion, heat, and temperature discussed in this essay emerged only in the period beginning about 1750. In any case, Galen proposed that equal amounts of boiling water and ice be combined to establish a "neutral" temperature, with four units of warmth above it and four degrees of cold below.


The great physicist Galileo Galilei (1564-1642) is sometimes credited with creating the first practical temperature measuring device, called a thermoscope. Certainly Galileowhether or not he was the firstdid build a thermoscope, which consisted of a long glass tube planted in a container of liquid. Prior to inserting the tube into the liquidwhich was usually colored water, though Galileo's thermoscope used wineas much air as possible was removed from the tube. This created a vacuum, and as a result of pressure differences between the liquid and the interior of the thermoscope tube, some of the liquid went into the tube.

But the liquid was not the thermometric mediumthat is, the substance whose temperature-dependent property changes the thermoscope measured. (Mercury, for instance, is the thermometric medium in most thermometers today.) Instead, the air was the medium whose changes the thermoscope measured: when it was warm, the air expanded, pushing down on the liquid; and when the air cooled, it contracted, allowing the liquid to rise.

It is interesting to note the similarity in design between the thermoscope and the barometer, a device for measuring atmospheric pressure invented by Italian physicist Evangelista Torricelli (1608-1647) around the same time. Neither were sealed, but by the mid-seventeenth century, scientists had begun using sealed tubes containing liquid instead of air. These were the first true thermometers.


Ferdinand II, Grand Duke of Tuscany (1610-1670), is credited with developing the first thermometer in 1641. Ferdinand's thermometer used alcohol sealed in glass, which was marked with a temperature scale containing 50 units. It did not, however, designate a value for zero.

English physicist Robert Hooke (1635-1703) created a thermometer using alcohol dyed red. Hooke's scale was divided into units equal to about 1/500 of the volume of the thermometric medium, and for the zero point, he chose the temperature at which water freezes. Thus, Hooke established a standard still used today; likewise, his thermometer itself set a standard. Built in 1664, it remained in use by the Royal Societythe foremost organization for the advancement of science in England during the early modern perioduntil 1709.

Olaus Roemer (1644-1710), a Danish astronomer, introduced another important standard. In 1702, he built a thermometer based not on one but two fixed points, which he designated as the temperature of snow or crushed ice, and the boiling point of water. As with Hooke's use of the freezing point, Roemer's idea of the freezing and boiling points of water as the two parameters for temperature measurements has remained in use ever since.

Temperature Scales


Not only did he develop the Fahrenheit scale, oldest of the temperature scales still used in Western nations today, but German physicist Daniel Fahrenheit (1686-1736) also built the first thermometer to contain mercury as a thermometric medium. Alcohol has a low boiling point, whereas mercury remains fluid at a wide range of temperatures. In addition, it expands and contracts at a very constant rate, and tends not to stick to glass. Furthermore, its silvery color makes a mercury thermometer easy to read.

Fahrenheit also conceived the idea of using "degrees" to measure temperature in his thermometer, which he introduced in 1714. It is no mistake that the same word refers to portions of a circle, or that exactly 180 degreeshalf the number in a circleseparate the freezing and boiling points for water on Fahrenheit's thermometer. Ancient astronomers attempting to denote movement in the skies used a circle with 360 degrees as a close approximation of the ratio between days and years. The number 360 is also useful for computations, because it has a large quantity of divisors, as does 180a total of 16 whole-number divisors other than 1 and itself.

Though it might seem obvious that 0 should denote the freezing point and 180 the boiling point on Fahrenheit's scale, such an idea was far from obvious in the early eighteenth century. Fahrenheit considered the idea not only of a 0-to-180 scale, but also of a 180-to-360 scale. In the end, he chose neitheror rather, he chose not to equate the freezing point of water with zero on his scale. For zero, he chose the coldest possible temperature he could create in his laboratory, using what he described as "a mixture of sal ammoniac or sea salt, ice, and water." Salt lowers the melting point of ice (which is why it is used in the northern United States to melt snow and ice from the streets on cold winter days), and, thus, the mixture of salt and ice produced an extremely cold liquid water whose temperature he equated to zero.

With Fahrenheit's scale, the ordinary freezing point of water was established at 32°, and the boiling point exactly 180° above it, at 212°. Just a few years after he introduced his scale, in 1730, a French naturalist and physicist named Rene Antoine Ferchault de Reaumur (1683-1757) presented a scale for which 0° represented the freezing point of water and 80° the boiling point. Although the Reaumur scale never caught on to the same extent as Fahrenheit's, it did include one valuable addition: the specification that temperature values be determined at standard sea-level atmospheric pressure.


With its 32-degree freezing point and its 212-degree boiling point, the Fahrenheit system is rather ungainly, lacking the neat orderliness of a decimal or base-10 scale. The latter quality became particularly important when, 10 years after the French Revolution of 1789, France adopted the metric system for measuring length, mass, and other physical phenomena. The metric system eventually spread to virtually the entire world, with the exception of English-speaking countries, where the more cumbersome British system still prevails. But even in the United States and Great Britain, scientists use the metric system. The metric temperature measure is the Celsius scale, created in 1742 by Swedish astronomer Anders Celsius (1701-1744).

Like Fahrenheit, Celsius chose the freezing and boiling points of water as his two reference points, but he determined to set them 100, rather than 180, degrees apart. Interestingly, he planned to equate 0° with the boiling point, and 100° with the freezing pointproving that even the most apparently obvious aspects of a temperature scale were once open to question. Only in 1750 did fellow Swedish physicist Martin Strömer change the orientation of the Celsius scale.

Celsius's scale was based not simply on the boiling and freezing points of water, but, specifically, those points at normal sea-level atmospheric pressure. The latter, itself a unit of measure known as an atmosphere (atm), is equal to 14.7 lb/in2, or 101,325 pascals in the metric system. A Celsius degree is equal to 1/100 of the difference between the freezing and boiling temperatures of water at 1 atm.

The Celsius scale is sometimes called the centigrade scale, because it is divided into 100 degrees, cent being a Latin root meaning "hundred." By international convention, its values were refined in 1948, when the scale was redefined in terms of temperature change for an ideal gas, as well as the triple point of water. (Triple point is the temperature and pressure at which a substance is at once a solid, liquid, and vapor.) As a result of these refinements, the boiling point of water on the Celsius scale is actually 99.975°. This represents a difference equal to about 1 part in 4,000hardly significant in daily life, though a significant change from the standpoint of the precise measurements made in scientific laboratories.


In about 1787, French physicist and chemist J. A. C. Charles (1746-1823) made an interesting discovery: that at 0°C, the volume of gas at constant pressure drops by 1/273 for every Celsius degree drop in temperature. This seemed to suggest that the gas would simply disappear if cooled to 273°C, which, of course, made no sense. In any case, the gas would most likely become first a liquid, and then a solid, long before it reached that temperature.

The man who solved the quandary raised by Charles's discovery was born a year after Charleswho also formulated Charles's lawdied. He was William Thomson, Lord Kelvin (1824-1907), and in 1848, he put forward the suggestion that it was molecular translational energy, and not volume, that would become zero at 273°C. He went on to establish what came to be known as the Kelvin scale.

Sometimes known as the absolute temperature scale, the Kelvin scale is based not on the freezing point of water, but on absolute zerothe temperature at which molecular motion comes to a virtual stop. This is 273.15°C (459.67°F), which in the Kelvin scale is designated as 0K. (Kelvin measures do not use the term or symbol for "degree.")

Though scientists normally use metric or SI measures, they prefer the Kelvin scale to Celsius, because the absolute temperature scale is directly related to average molecular translational energy. Thus, if the Kelvin temperature of an object is doubled, this means that its average molecular translational energy has doubled as well. The same cannot be said if the temperature were doubled from, say, 10°C to 20°C, or from 40°C to 80°F, since neither the Celsius nor the Fahrenheit scale is based on absolute zero.


The Kelvin scale is, however, closely related to the Celsius scale, in that a difference of 1 degree measures the same amount of temperature in both. Therefore, Celsius temperatures can be converted to Kelvins by adding 273.15. There is also an absolute temperature scale that uses Fahrenheit degrees. This is the Rankine scale, created by Scottish engineer William Rankine (1820-1872), but it is seldom used today: scientists and others who desire absolute temperature measures prefer the precision and simplicity of the Celsius-based Kelvin scale.

Conversion between Celsius and Fahrenheit figures is a bit more challenging. To convert a temperature from Celsius to Fahrenheit, multiply by 9/5 and add 32. It is important to perform the steps in that order, because reversing them will produce a wrong answer. Thus, 100°C multiplied by 9/5 or 1.8 equals 180, which, when added to 32 equals 212°F. Obviously, this is correct, since 100°C and 212°F each represent the boiling point of water. But, if one adds 32 to 100°, then multiplies it by 9/5, the result is 237.6°Fan incorrect answer.

For converting Fahrenheit temperatures to Celsius, there are also two steps, involving multiplication and subtraction, but the order is reversed. Here, the subtraction step is performed before the multiplication step: thus, 32 is subtracted from the Fahrenheit temperature, then the result is multiplied by 5/9. Beginning with 212°F, if 32 is subtracted, this equals 180. Multiplied by 5/9, the result is 100°Cthe correct answer.

One reason the conversion formulae use fractions instead of decimal fractions (what most people simply call "decimals") is that 5/9 is a repeating decimal fraction (0.55555.) Further more, the symmetry of 5/9 and 9/5 makes memorization easy. One way to remember the formula is that F ahrenheit is multiplied by a f ractionsince 5/9 is a real fraction, whereas 9/5 is actually a whole number plus a fraction.


As discussed earlier, with regard to the early history of the thermometer, it is important that the glass tube be kept sealed; otherwise, atmospheric pressure contributes to inaccurate readings, because it influences the movement of the thermometric medium. Also important is the choice of the thermometric medium itself.

Water quickly proved unreliable, due to its unusual properties: it does not expand uniformly with a rise in temperature, or contract uniformly with a lowered temperature. Rather, it reaches its maximum density at 39.2°F (4°C), and is less dense both above and below that temperature. Therefore, alcohol, which responds in a much more uniform fashion to changes in temperature, took its place.


Alcohol is still used in thermometers today, but the preferred thermometric medium is mercury. As noted earlier, its advantages include a much higher boiling point, a tendency not to stick to glass, and a silvery color that makes its levels easy to gauge visually. Like alcohol, mercury expands at a uniform rate with an increase in temperature: hence, the higher the temperature, the higher the mercury stands in the thermometer.

In a typical mercury thermometer, mercury is placed in a long, narrow sealed tube called a capillary. The capillary is inscribed with figures for a calibrated scale, usually in such a way as to allow easy conversions between Fahrenheit and Celsius. A thermometer is calibrated by measuring the difference in height between mercury at the freezing point of water, and mercury at the boiling point of water. The interval between these two points is then divided into equal increments180, as we have seen, for the Fahrenheit scale, and 100 for the Celsius scale.


Faster temperature measures can be obtained by thermometers using electricity. All matter displays a certain resistance to electrical current, a resistance that changes with temperature. Therefore, a resistance thermometer uses a fine wire wrapped around an insulator, and when a change in temperature occurs, the resistance in the wire changes as well. This makes possible much quicker temperature readings than those offered by a thermometer containing a traditional thermometric medium.

Resistance thermometers are highly reliable, but expensive, and are used primarily for very precise measurements. More practical for everyday use is a thermistor, which also uses the principle of electric resistance, but is much simpler and less expensive. Thermistors are used for providing measurements of the internal temperature of food, for instance, and for measuring human body temperature.

Another electric temperature-measurement device is a thermocouple. When wires of two different materials are connected, this creates a small level of voltage that varies as a function of temperature. A typical thermocouple uses two junctions: a reference junction, kept at some constant temperature, and a measurement junction. The measurement junction is applied to the item whose temperature is to be measured, and any temperature difference between it and the reference junction registers as a voltage change, which is measured with a meter connected to the system.


A pyrometer also uses electromagnetic properties, but of a very different kind. Rather than responding to changes in current or voltage, the pyrometer is a gauge that responds to visible and infrared radiation. Temperature and color are closely related: thus, it is no accident that greens, blues, and purples, at one end of the visible light spectrum, are associated with coolness, while reds, oranges, and yellows at the other end are associated with heat. As with the thermocouple, a pyrometer has both a reference element and a measurement element, which compares light readings between the reference filament and the object whose temperature is being measured.

Still other thermometers, such as those in an oven that tell the user its internal temperature, are based on the expansion of metals with heat. In fact, there are a wide variety of thermometers, each suited to a specific purpose. A pyrometer, for instance, is good for measuring the temperature of an object that the thermometer itself does not touch.


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The temperature, defined as 0K on the Kelvin scale, at which the motion of molecules in a solid virtually ceases.


A scale of temperature, sometimes known as the centigradescale, created in 1742 by Swedish astronomer Anders Celsius (1701-1744). The Celsius scale establishes the freezing and boiling points of water at 0° and 100°, respectively. To convert a temperature from the Celsius to the Fahrenheit scale, multiply by 9/5 and add 32. The Celsius scale is part of the metric system used by mostnon-English speaking countries today. Though the worldwide scientific community uses the metric or SI system for most measurements, scientists prefer the related Kelvin scale.


The oldest of the temperature scales still used in Westernnations today, created in 1714 by German physicist Daniel Fahrenheit (1686-1736). The Fahrenheit scale establishes the freezing and boiling points of water at 32° and 212° respectively. To convert a temperature from the Fahrenheit to the Celsius scale, subtract 32 and multiply by 5/9. Most English-speaking countries use the Fahrenheitscale.


Internal thermal energy that flows from one body of matter to another.


Established by William Thomson, Lord Kelvin (1824-1907), the Kelvin scale measures temperature in relation to absolute zero, or 0K.(Units in the Kelvin system, known as Kelvins, do not include the word or symbol for degree.) The Kelvin and Celsius scales are directly related; hence, Celsius temperatures can be converted to Kelvins by adding 273.15. The Kelvin scale is used almost exclusively by scientists.


The energy that an object possesses by virtue of its motion.


The kinetic energy in a system produced by the movement of molecules in relation to one another.


In physics, the term "system" usually refers to any set of physical interactions, or any material body, isolated from the rest of the universe. Anything outside of the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment.


A measure of the average kinetic energyor molecular translational energy in a system. Differences in temperature determine the direction of internal energy flow between two systems when heat is being transferred.


Heat energy, a form of kinetic energy produced by the movement of atomic or molecular particles. The greater the movement of the separticles, the greater the thermal energy.


The statethat exists when two systems have the same temperature. As a result, there is no exchange of heat between them.


The study of the relationships between heat, work, and energy.


A substance whose properties change with temperature. A mercury or alcohol thermometer measures such changes.


A device that gauges temperature by measuring a temperature-dependent property, such as the expansion of a liquid in a sealed tube, or resistance to electric current.


The temperature and pressure at which a substance is at once asolid, liquid, and vapor.


Space entirely devoid of matter, including air.

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The concept of temperature has two related, but different, interpretations. On a general level, temperature is associated with the sense of hot and cold. If you put your finger in a pan of hot water, heat energy flows from the water to your finger; you say that the water is at a higher temperature than that of your finger. If you put your finger in a glass of ice water, heat energy flows as heat away from your finger.

The direction of heat energy flow is the basis of one definition of temperature. Temperature is the property of objectsor more generally of systemsthat determines the direction of heat energy flow when the objects are put in direct contact with each other. Energy flows as heat from objects at a higher temperature to ones at a lower temperature. When heat energy ceases to flow, the objects are at the same temperature and are said to be in thermal equilibrium.

The second definition of temperature is more rigorous. It deals with the factors that are responsible for an object's being warm or hot on the one hand or cool or cold on the other. This definition is based on the behavior of the particles (atoms, molecules, ions, etc.) of which matter is made. On this level, temperature can be defined as the total kinetic energy of the particles of which a material is made.

Kinetic energy is the motion of particles. Particles that are rotating rapidly on their axes, vibrating back and forth rapidly, or traveling rapidly through space have a large amount of kinetic energy. Particles that are moving slowly have relatively little kinetic energy.

Words to Know

Absolute temperature scale: A temperature scale that has the lowest possible temperatureat which all molecular motion ceasesset at zero.

Absolute zero: The lowest possible temperature at which all molecular motion ceases. It is equal to 459°F (273°C).

Kinetic energy: Energy of an object or system due to its motion.

Pyrometer: A device for obtaining temperature by measuring the amount of radiation produced by an object.

Resistance thermometer: A device for obtaining temperature by measuring the resistance of a substance to the flow of an electrical current.

Thermometer: A device for obtaining temperature by measuring a temperature-dependent property (such as the height of a liquid in a sealed tube) and relating this to temperature.

From this perspective, a glass of warm water has a high temperature because the molecules of the water are moving rapidly. The molecules of water in a glass of cool water, by comparison, are moving more slowly.

Temperature measurement: Thermometers

Thermometers are devices that register the temperature of a substance relative to some agreed upon standard. For example, a thermometer that reads 32°F (0°C) is measuring a temperature equal to that of ice in contact with pure water.

Thermometers use changes in certain physical or electrical properties to detect temperature variations. The most common kind of thermometer consists of a liquidusually mercury or alcoholsealed in a narrow tube. When the thermometer is placed in contact with a substance, heat travels into or out of the thermometer. If heat leaves the thermometer, the sealedin liquid is cooled and it contracts (takes up less space); the level of the liquid in the thermometer falls. If heat enters the thermometer, the liquid is warmed and it expands; the level of the liquid in the thermometer rises.

A resistance thermometer is based on the fact that all things resist the flow of an electric current to some degree. Furthermore, such resistance changes with temperature. In general, the higher the temperature of a substance, the more it resists the flow of an electric current. This principle can be used to measure the temperature of a substance by observing the extent to which it resists the flow of an electric current.

Another type of thermometer is known as a pyrometer. A pyrometer is a device that detects visible and infrared radiation given off by an object, then converts that information to a temperature reading.

Temperature measurement: Scales

In order to establish a scale against which temperatures can be measured, one first has to select two fixed points from which to begin. Historically, those two points have been the boiling point and freezing point of water. The two points were chosen because water is the most abundant compound on Earth, and finding its boiling and freezing points is relatively easy.

One way of making a thermometer, then, is to begin with a narrow tube that contains a liquid and is sealed at both ends. The tube is then immersed in boiling water, and the highest point reached by the liquid is marked in some appropriate way. Next, the tube is immersed in a mixture of ice and water, and the lowest point reached by the liquid is marked in a similar way. The distance between the lowest point and highest point is then divided into equal sections. The numbers assigned to the lowest and highest point on the thermometerand the form of dividing the range between themis what distinguishes one system of measuring temperature from another.

In the early 1700s, for example, German physicist Gabriel Daniel Fahrenheit (16861736) decided to assign to the freezing point of water a temperature value of 32 and to the boiling point of water a temperature value of 212. He then divided the distance between these two points into 180 equal divisions, each equal to one degree of temperature. The Fahrenheit (F) system of measuring temperature is still in use today in the United States.

A somewhat more logical system of defining the temperatures on a thermometer was suggested in 1742 by Swedish astronomer Anders Celsius (17011744). Celsius suggested assigning the values of 0 and 100 to the freezing point and boiling point of water and dividing the distance between these two into 100 equal parts. The Celsius system is now used throughout the scientific community and in all countries of the world except the United States and Burma.

Both Fahrenheit and Celsius temperature scales have one important inherent drawback: in both cases, negative temperatures can exist. The freezing point of carbon dioxide (dry ice), for example, is 110°F (78.5°C). But recall the definition of temperature as a measure of the average kinetic energy of the particles that make up a substance. What meaning can be assigned, then, to a negative temperature reading? There is no such thing as negative energy in systems with which we are familiar.

To remedy this problem, a third temperature scale was invented in 1848 by English physicist William Thomson (18241907). Thomson set the lowest point on the temperature scale as the lowest possible temperature, absolute zero. Absolute zero is defined as the temperature at which all motion of all particles would cease, a condition in which heat would be absent and, hence, a substance had no temperature. Theoretical calculations suggested to Thomson that the Celsius temperature corresponding to that condition was about 273°C (459°F). Absolute zero, then, was set at this temperature and assigned the value 0 K. The unit K in this measurement stands for Kelvin, the unit of measure in the absolute temperature system. The term Kelvin comes from William Thomson's official title, Lord Kelvin.

The relationship among the three temperature scales is as follows:

°C = 5/9(°F 32)

°F = 9/5(°C) + 32

K = °C + 273

°C = K 273

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In everyday terms, temperature is a measure of the "hotness" or "coldness" of a substance. More technically, temperature indicates the direction in which energy flows (as heat) when two objects are in thermal contact: energy flows as heat from a high temperature region to a low temperature region. In other words, temperature is simply an indicator of the expected direction of flow of energy as heat.

Temperature is not heat. Heat is energy in transition; temperature is the signpost of the expected direction of that transition. A large quantity of energy can flow as heat from one region to another even though the temperature difference between the regions is minute.

Temperature is not energy. A very large, cold block of metal will have a low temperature but may contain a very large amount of energy. A small block of the same material having the same temperature will contain less energy. This distinction is expressed by saying that temperature is an intensive property, a property independent of the size of the sample; whereas energy content is an extensive property, a property that does depend on the size of the sample. Thus, a sample taken from a tank of hot water will have the same temperature regardless of the size of the sample, but the energy content (more formally, the internal energy) of a large sample is greater than that of a small sample.

At a molecular level, the temperature of a system indicates the distribution of "populations" of energy levels within the system: the higher the temperature, the greater the proportion of molecules in a state of high energy. If the numbers of molecules in two energy states, separated by an energy difference ΔE, are N upper and N lower, then the temperature is

T = (ΔE /k ) ln(N lower/N upper)   (1)

where k is Boltzmann's constant, a fundamental constant of nature. We see that the greater the ratio N lower/N upper for a given energy difference, the higher the temperature. This molecular interpretation has a special significance in cases in which the only contribution to the overall energy is kinetic energy, which is the case in a perfect (ideal) gas. In that case, high temperature corresponds to a higher average speed of the molecules and a wider range of speeds in the sample. The average speed c of molecules of mass m at a temperature T is

c = (8kT /π m )½   (2)

and so the average speed increases with the square root of the temperature.

Temperature is measured with a thermometer, a device in which a physical property of some component of the device changes when the device is put in thermal contact with a sample. That property may be the volume of a liquid (as in a mercury-in-glass thermometer) or an electrical property such as resistance. Electronic probes based on resistance changes in a semiconductor material are also used to measure temperature.

Three scales of temperature are still commonly encountered. The Fahrenheit scale is used in the United States for domestic purposes. On this

scale, the freezing point of water is 32°F and its boiling point is 212°F. This scale has been discarded by virtually all other countries in favor of the Celsius scale, which is used for all scientific work. On the Celsius scale, the freezing point of water corresponds to 0°C and the boiling point corresponds to 100°C. A more fundamental scale is the Kelvin scale, which sets 0 at the absolute zero of temperature (corresponding to 273.15°C), and adopts a scale in which the triple point of water (the temperature at which ice, water, and water vapor coexist at equilibrium ) is exactly 273.16 K. This scale ensures that the magnitude of the kelvin (as the unit for the Kelvin scale is called) is the same as that of the Celsius degree.

The Kelvin scale is used to express the thermodynamic temperature, denoted T, with T = 0 as the lowest possible temperature (when all motion has ceased). Temperatures on the Celsius and Fahrenheit scales are denoted θ (theta). Two important conversions are:

θ /ºC = 5/9(θ /ºF 32)   (3)

T /K = θ /ºC + 273.15   (4)

In chemistry, it is often necessary to keep a system at a constant temperature, for otherwise observations and measurements would provide a reading that was an average of a temperature-dependent property, such as reaction rate. One way to achieve a constant temperature is to immerse the system in a water bath containing a large volume of water, the temperature of which is controlled by a heater and a thermostat. A thermostat is a device for switching a current on and off according to whether the temperature of the system is above or below a selected value. It incorporates a temperature probe (a thermometer with an electric output) and electronic devices for interpreting the temperature and effecting the switching. The same principle is the basis of the thermostat that is used in homes.

The chemical effects of greater temperature include changes in the rate of reaction and the position of chemical equilibrium. Almost all reactions proceed more rapidly at higher temperatures because the molecules (in the gas phase ) collide more vigorously at higher temperatures. A thermodynamic consequence of changing temperature is that the equilibrium constant of an exothermic reaction decreases as the temperature is raised, so reactants are more favored at low temperatures than at high. This dependence is sometimes referred to as Le Chatelier's principle, but it is better to regard it as a consequence of thermodynamics and in particular of the second law of thermodynamics.

Although T = 0 is the lowest attainable temperature, it is possible to achieve negative temperatures. This seemingly paradoxical remark is resolved as follows. When a system has only two energy levels, all finite temperatures correspond to a distribution of populations in which more molecules occupy the lower state than the upper. However, it is possible by artificial means to invert the populations, so that briefly there would be more molecules in the upper state than the lower. It follows from equation 1 that T is then negative.

The thermodynamic justification for introducing the temperature into science is the Zeroth Law, which states that if system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then A and C would also be in thermal equilibrium with each other, if they were put in contact. The third law of thermodynamics is also relevant here: it states that absolute zero (T = 0) is not attainable in a finite number of steps.

see also Chemistry and Energy; Energy; Heat; Physical Chemistry; Thermodynamics.

Peter Atkins


Atkins, Peter, and de Paula, Julio (2002). Atkins' Physical Chemistry, 7th edition. New York: Oxford University Press.

Smith, Crosbie (1998). The Science of Energy: A Cultural History of Energy Physics in Victorian Britain. Chicago: University of Chicago Press.

Tipler, Paul A. (1999). Physics for Scientists and Engineers, 4th edition. New York: W. H. Freeman; Worth Publishers.

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temperature, measure 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°Re and the boiling point at 80°Re. 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. See heat; thermodynamics.

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temperature In physics, measure of the hotness or coldness of an object. Strictly, it describes the number of energy states available to a substance or system. Two objects in thermal contact initially exchange heat energy but eventually arrive at thermal equilibrium. At equilibrium, the most probable distribution of energy states among the atoms and molecules of the objects has been attained. At high temperatures, the number of energy states available to the atoms and molecules of a system is large; at lower temperatures, fewer states are available. At a sufficiently low temperature, all parts of the system are at their lowest energy levels, the absolute zero of temperature.

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tem·per·a·ture / ˈtemp(ə)rəchər; -ˌchoŏr/ • n. the degree or intensity of heat present in a substance or object, esp. as expressed according to a comparative scale and shown by a thermometer or perceived by touch. ∎  Med. the degree of internal heat of a person's body: I'll take her temperature. ∎  inf. a body temperature above the normal; fever: he was running a temperature. ∎  the degree of excitement or tension in a discussion or confrontation: the temperature of the debate was lower than before.

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"temperature." The Oxford Pocket Dictionary of Current English. . 30 Apr. 2017 <>.

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temperature In biology, intensity of heat. In warm-blooded (homeothermic) animals, body temperature is maintained within narrow limits regardless of the temperature of their surroundings. This is accomplished by muscular activity, the operation of cooling mechanisms, such as vasodilation, vasoconstriction and sweating, and metabolic activity. In humans, normal body temperature is c.36.9°C (98.4°F), but this varies with degree of activity. In so-called cold-blooded (poikilothermic) animals, body temperature varies between wider limits, depending on the temperature of the surroundings.

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The purpose of this writing is to provide background information concerning the relationship between water temperature and fish. Since fish inhabit only a small fraction of a body of water, it is imperative to be able to locate these areas; otherwise, it is a waste of your time to fish in areas void of fish. Knowledge of temperatures influence on fish can help you both locate and catch them.

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