Thermometers

Type of physical science: Classical physics

Field of study: Thermodynamics

A thermometer is a device used to measure temperature and thus assign a numerical value to the degree of hotness or coldness of matter. Temperature is an essential parameter used to understand a wide variety of scientific phenomena and to characterize chemical and physical properties of matter.

Overview

Temperature is a quantitative measurement of the degree of hotness or coldness of a collection of matter. Thermometers are used to determine temperature by measuring some physical property that varies with temperature. One important property that varies with temperature is the direction of heat flow. The natural direction of heat flow between two collections of matter or bodies is from the warmer to the cooler. When two bodies are in contact, heat will flow from the hotter to the cooler until the two bodies are in thermal equilibrium. At this time, no additional heat will flow, and the two bodies are said to have an identical temperature.

The observation that two bodies, each in thermal equilibrium with a third body, are in thermal equilibrium with each other is sometimes referred to as the "zeroth law of thermodynamics." This universal observation is not dependent on the composition of the body or the state of matter--that is, whether it is a solid, liquid, or gas. The zeroth law observation is important because it allows scientists to define a universal temperature scale. One can imagine the third body of matter to be a device called a thermometer. The thermometer can be used not only to establish that two different collections of matter are at the same temperature but also to establish numerical values of temperature that are independent of the state of matter or its composition. In this sense, temperature is a universal property.

The problem involved in measuring temperature is to establish a specific temperature scale and to have a reproducible method of determining the numerical values of temperature.

Yet, to understand what a thermometer represents when a particular temperature is reported, it is necessary to understand something about the connection between atomic motion and temperature.

The temperature of a collection of matter is dependent on the collective atomic motion.

An increase in atomic motion is associated with an increase in temperature, and a slowing of atomic motion is associated with a decrease in temperature. This atomic motion may be from vibration of atoms or from the rotation or translation (moving from place to place) of molecules.

The total energy caused by the constant random motion of a collection of molecules is the heat, whereas the temperature is the average energy of molecular motion. Thus, a large collection of matter may have a greater heat content than a smaller collection of matter, even though both have the same temperature. In solids, the vibrational motion of atoms stores heat energy, and the vibrational energy can be increased or decreased as the temperature is increased or decreased.

The average speed of motion of the molecules in a gas is proportional to the temperature of the collection of molecules. The average speed of the molecules increases as the temperature is increased, and the average speed of molecules decreases as the temperature is decreased.

With this connection between atomic motion and temperature made clear, it becomes obvious that there is a lowest possible temperature. At absolute zero, atomic motion is at the lowest possible value and the randomness or disorder in an ordered solid is 0. Entropy is a measure of this disorder, and the entropy of a perfect crystalline solid is 0 at absolute zero. The thermodynamic temperature scale is based on a heat flow ratio proportional to temperature ratio and the realization that there is a lowest possible temperature. In the thermodynamic temperature scale, there are no negative values. The low end of the scale is fixed at absolute zero, where the temperature is exactly 0 Kelvin.

The Kelvin scale is one temperature scale, but there are other scales that are used to quantify the hotness or coldness of matter. The three most important temperature scales are Fahrenheit, Celsius, and Kelvin. The Celsius and Kelvin scales are the most widely used in science. A unit on the Fahrenheit and Celsius scales is referred to as a degree; a unit on the Kelvin scale is referred to as a Kelvin.

It is most convenient to compare these temperature units by comparing the values each scale assigns to the melting point and boiling point of water under a pressure of exactly 1 atmosphere. On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees. On the Celsius scale, water freezes at 0 degrees and boils at 100 degrees. On the Kelvin scale, the freezing point of water is 273.15 Kelvins and the boiling point of water is 373.15 Kelvins. To convert Celsius degrees to Kelvins, one simply adds 273.15. For example, 20.00 degrees Celsius is equivalent to 293.15 Kelvins.

The thermodynamic temperature scale is based on defining the triple point of water (where liquid, solid, and gas all coexist) at exactly 273.16 Kelvins, or 0.01 degree Celsius. The Kelvin scale is the most useful temperature scale in science because there are no negative temperatures. Absolute zero has a value of exactly 0 on the Kelvin scale. On the Celsius and Fahrenheit scales, absolute zero has a value of -273.15 and -459.67, respectively.

One method to determine absolute temperature values is through the use of a gas thermometer, which is based on how temperature affects the pressure of a gas. The gas is considered to be an ideal gas, meaning that there are no interactions between gas molecules.

Since all gases have some attractions between molecules, it is necessary to correct for nonideality.

Helium gas behaves most like an ideal gas and so it is the best choice for a gas thermometer. For a fixed volume and a fixed amount of helium, the pressure is measured with a mercury manometer at various temperatures. A manometer is a device used to measure pressure by measuring the height of a column of mercury supported by that pressure. Pressure can be used to measure the temperature because for an ideal gas the height of the column of mercury is proportional to the pressure, and according to the ideal gas law, the pressure is proportional to the temperature. As the temperature is changed, one must correct for gas imperfections and the expansion and the contraction of the gas container.

Because the use of a gas thermometer to obtain accurate temperature values is an extremely tedious process, it is normally used only to determine a few fixed point temperatures.

Fixed point temperatures do not vary, because they are based on equilibrium between two or three different phases or states of matter. A triple point is based on an equilibrium between solid, liquid, and vapor. A melting point is based on an equilibrium between solid and liquid, and a boiling point is based on an equilibrium between a liquid and its vapor.

The International Practical Temperature Scale is selected to correspond to the thermodynamic temperature scale by using a few fixed points as reference values to define the scale. Useful fixed points under a pressure of exactly 1 atmosphere include the triple points of hydrogen (13.81 Kelvins), oxygen (54.361 Kelvins), and water (273.16 Kelvins); the boiling points of hydrogen (20.28 Kelvins), neon (27.102 Kelvins), oxygen (90.188 Kelvins), and water (373.15 Kelvins); and the melting points of zinc (692.73 Kelvins), silver ,.23 Kelvins), and gold ,.58 Kelvins). A platinum resistance thermometer is used from -259.34 degrees Celsius (13.81 Kelvins) to 630.74 degrees Celsius (903.89 Kelvins), the melting point of the metal antimony, to measure values on the International Practical Temperature Scale. The temperature scales of most other thermometers are fixed by comparing them to a standard platinum resistance thermometer.

A variety of thermometers have been developed, including mercury, Bourdon tube, bimetallic, thermocouple, resistance, and thermistor. Liquid expansion thermometers are the most common type of thermometer and normally use mercury as the working liquid. (Alcohol can be used in regions where the temperature goes below the freezing point of mercury, 234.3 Kelvins, or -38.85 degrees Celsius.) A bulb containing mercury is connected to a tube partially filled with mercury. As the temperature increases, the liquid mercury expands to fill a greater volume and rises in the tube. As the temperature decreases, the liquid mercury contracts to fill a smaller volume and goes down in the tube. Conveniently, the expansion and contraction of mercury with increasing and decreasing temperature are fairly linear over a broad range of temperature, so that the height of the mercury column in the tube is directly related to the temperature of the environment in which the thermometer is placed. A scale marked on the glass tube is used to determine the numerical value of the temperature.

Another type of liquid expansion thermometer is a Bourdon tube thermometer. A Bourdon tube is a curved tube made of a flexible metal that is filled with a liquid. As the temperature is increased, the liquid expands and the curved tube straightens out slightly. The temperature is indicated by a pointer or recorded by a marker attached to the end of the tube.

A bimetallic thermometer is based on the expansion of solids. Two metals that have different coefficients of expansion, such as brass and iron, are connected together to make a thin, spiral bar. Temperature changes cause the spiral bar to bend into a tighter spiral or relax into a looser spiral. A pointer attached to the end of the bar indicates the temperature along a scale marked above the pointer.

The two main types of electrical thermometers depend on monitoring either changes of voltage potential or changes in electrical resistance. A thermocouple is used to measure temperature by monitoring potential. In a thermocouple, two different wires, such as copper and constantan (a copper and nickel alloy), are connected together to make two junctions. One junction is kept at a known temperature, such as in melting ice, which is exactly 0 degrees Celsius. This junction is known as the reference junction, and the temperature of the other junction is measured relative to it. When the second junction is at a temperature different from that of the reference junction, then there is a voltage potential across the two thermocouples. As the temperature difference increases, the potential difference between the two thermocouple junctions also increases. This potential is measured with a voltmeter capable of measuring millivolts or better. Either standard tables are used to convert millivolt potential difference to a temperature difference or a conversion to a temperature scale is done by an integrated circuit and the temperature is read directly on a digital scale or on a computer monitor.

Resistance thermometers, the other type of electrical thermometer, measure electrical resistance. Thermistors (a contraction from "thermally sensitive resistors") are metal oxides that have an extremely large decrease in resistance to electrical current flow as the temperature is increased. Thermistors such as nickel oxide and manganese oxide are about ten times as sensitive as metal resistors to temperature change. Numerous complications in measuring accurate resistances are eliminated by switching from metallic to metallic oxide resistors. Thermistors are convenient to use, but the resistance characteristics tend to change with use and therefore need to be recalibrated periodically.

Applications

The accurate measurement of temperature is of vital interest in a wide array of scientific and engineering endeavors. Temperature is important in understanding weather patterns and the extent to which the planet is being subjected to global warming as a result of the greenhouse effect. Accurate temperature measurements are a routine part of determining whether a patient has a fever that could indicate infection or illness. Clinical thermometers are used to measure the internal temperature of the human body. Normal body temperature is 98.6 degrees Fahrenheit, or 37 degrees Celsius.

Temperature is important in determining the utility of new superconducting materials, in monitoring energy production with calorimetric experiments such as the early cold fusion work, in determining properties such as boiling point and melting point of new molecules and compounds, in following reaction conditions in chemical and industrial manufacturing, and in maintaining comfortable but energy-efficient living conditions in buildings and vehicles.

Since many different types of temperature measurements are routinely used, many different methods of measuring temperature have been developed. All these different types of thermometers accomplish their goals, not by measuring temperature directly but rather by measuring a physical property that changes in a known way with temperature. Temperature affects a wide variety of physical, electrical, magnetic, and optical properties.

Mercury thermometers are the most common type of thermometer. Most household thermometers are mercury thermometers. These devices are used to measure indoor and outdoor temperature. Knowledge of outdoor temperature is important in determining how to dress properly for either cold or warm days. On very cold days, there is the danger of hypothermia if a person is exposed to cold temperatures outside for an extended period and his or her internal body temperature drops. On hot days, it is important to drink fluids so as not to become dehydrated. Measuring the temperature accurately helps one to be prepared for whatever weather is encountered.

Measuring indoor temperature is important in maintaining comfortable surroundings while being as efficient in energy usage as possible. In a bimetallic thermometer, temperature changes cause the spiral bar made of two kinds of metal to bend into a tighter spiral or relax into a looser spiral. These bimetallic thermometers are used commonly in thermostats to control indoor temperatures. The thermostat is set on the desired temperature; as the bimetallic bar in the thermostat changes position, it causes an electrical contact to be made or broken. In this manner, heating or cooling devices are turned on or off, and a constant indoor temperature is maintained.

Thermocouples and resistance thermometers are useful for remote monitoring, since the temperature probe holding the thermocouple junction or resistor can be far from the site where the potential or resistance is measured. In many industrial plants, it is important to be able to monitor the temperature continuously at remote locations throughout the facility. In chemical engineering complexes where large-scale chemical reactions are carried out, it is necessary to be able to follow the temperature of the reaction to make sure that the reaction is not going too rapidly and that not too much heat is being produced. In nuclear power plants, it is also important to follow the temperature closely to ensure that the reactor is operating at the proper conditions.

Cooking is another activity in which thermocouple and resistance thermometers prove to be essential. In any type of cooking, it is important to maintain careful control over the temperature of the food. Electrical monitoring and control of oven temperature allows one to ensure that food is cooked to a proper temperature and for the correct length of time.

Optical pyrometers are used to determine temperatures above 900 Kelvins, where the wavelength or color of a hot object changes in a known way with temperature. The electromagnetic radiation or light of a predictable wavelength given off by heated objects is known as black body radiation. Optical pyrometers operate by using a photometer to measure the intensity of light given off at a particular wavelength. Optical pyrometers are useful in iron and steel manufacture, where molten metals must be maintained and poured from one container to another. It is essential to maintain the hot liquid metal at the correct temperature, and hand-held pyrometers allow the operator to check the temperature of the molten metal without touching it directly.

There are various other specialized devices for measuring temperature, including, but not limited to, quartz thermometers, in which the resonance frequency of a vibrating quartz crystal is changed with temperature; integrated-circuit transducers, which provide a linear output with temperature change; and vapor pressure thermometers, in which the vapor pressure of a liquid or solid that is dependent on temperature is measured.

Context

A thermoscope is a device that can measure changes in temperature without accurately measuring actual temperatures. The first thermoscope was made by Galileo in 1603 by observing changes in the height of water in a glass tube inserted into a container of water. Galileo's thermoscope responded to temperature changes but was also affected by changes in air pressure.

About fifty years later, Ferdinand II, the Grand Duke of Tuscany, and Robert Boyle independently realized that effects resulting from air pressure could be eliminated by sealing the water into a straight glass tube. With air pressure effects removed, the first true thermometers were made.

Early thermometers used water or alcohol as the working liquid. The first modern mercury thermometer was made in 1714 by Gabriel Daniel Fahrenheit, who used the expansion of mercury in a sealed glass tube as a measure of temperature. His zero point was obtained by mixing equal weights of snow and sodium chloride salt. On the Fahrenheit scale, water freezes at 32 degrees Fahrenheit and boils exactly 180 degrees higher, at 212 degrees Fahrenheit. In 1742, Anders Celsius developed a scale with one hundred divisions between the freezing and boiling points of water. It was Sir William Thomson (Lord Kelvin) who first suggested a scale using degrees with the same spacing as Celsius but starting at absolute zero.

In 1858, a German physician, Karl August Wunderlich, introduced the practice of determining body temperature as an indicator of illness with a mercury thermometer. The clinical thermometer was invented about ten years later by Sir Thomas Clifford Allbutt. In a clinical thermometer, the liquid mercury rises in the glass tube at elevated temperatures but does not return to lower values, because of a constriction in the glass tube that causes the mercury in the bottom bulb to separate from the mercury in the tube as the temperature is lowered. The bulb and tube mercury is rejoined by vigorously shaking the tube downward.

While liquid expansion thermometers continue in widespread use, modern measurements of temperature can also utilize bimetallic, Bourdon tube, thermocouple, resistance, and pyrometric thermometers, depending on the temperature range and application of interest. In addition to providing information about the daily weather, the measurement of temperature remains important in medicine, engineering, and science. As new frontiers of low and high temperature and properties of matter are explored, the careful and exact measurement of temperature will continue to play a central role in scientific observations.

Principal terms

ABSOLUTE ZERO: the lowest possible temperature (assigned a value of 0 on the Kelvin scale) and the temperature at which atomic motion and randomness of atomic arrangement are at a minimum

HEAT: the energy resulting from random motion of molecules, atoms, or subatomic particles

INTERNATIONAL PRACTICAL TEMPERATURE SCALE: the standard temperature scale based on a platinum resistance thermometer or optical pyrometer and one to which other thermometers are referenced

PYROMETER: a device used to determine temperatures above 900 Kelvins by measuring the intensity of light emitted by a hot object at a known wavelength

RESISTANCE THERMOMETER: a device used to measure temperature by measuring the change in resistance to electrical flow with temperature

TEMPERATURE: the average energy resulting from random motion of molecules, atoms, or subatomic particles

THERMOCOUPLE: a device used to measure temperature by monitoring the potential between two different metals

TRIPLE POINT: the unique temperature where a substance's solid, liquid, and vapor phases all exist together in equilibrium

ZEROTH LAW OF THERMODYNAMICS: the observation that two bodies, each in thermal equilibrium with a third body, are in thermal equilibrium with each other

Bibliography

Asimov, Isaac. ASIMOV'S NEW GUIDE TO SCIENCE. New York: Basic Books, 1984. Although this book presents an overview of all the biological and physical sciences, it has great detail for historical development and contributions of individual scientists. It has good sections on history of low-temperature developments and history of developments in measuring temperature.

Atkins, P. W. THE SECOND LAW. New York: W. H. Freeman, 1984. This book presents a lively discussion of the second law of thermodynamics and the intellectual origins and implications of the tendency toward randomness and disorder. Includes discussion of connections between temperature, heat, and absolute zero. Excellent color drawings aid significantly in giving a clear understanding of these phenomena.

Fenn, John B. ENGINES, ENTROPY, AND ENERGY. New York: W. H. Freeman, 1982. A cartoon caveman appears throughout chapters to help explain with wit and humor energy, heat, and entropy, and how these ideas affect real processes occurring in machines or in nature. Although equations are used throughout, the discussions make these concepts accessible to those without science and mathematics background.

Spielberg, Nathan, and Bryon D. Anderson. SEVEN IDEAS THAT SHOOK THE UNIVERSE. New York: John Wiley & Sons, 1987. This is an examination of the key ideas in physics and includes, in a section on energy, discussions on how heat, motion, and temperature are related. The section on entropy discusses the second and third laws of thermodynamics, temperature, and absolute zero.

Weber, Robert L. HEAT AND TEMPERATURE MEASUREMENT. Englewood Cliffs, N.J.: Prentice-Hall, 1950. This book includes tremendous detail on all manner of methods used to measure temperature and heat flow. Although parts are more technical, it is worth the read to learn something of how much is involved in careful, classic measurements of temperature.

Temperature scales compared

The Behavior of Gases

Liquefaction of Gases

Thermal Properties of Solids

Thermal Properties of Matter

Laws of Thermodynamics