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Terms: Temperature Thermometer Galileo Thermometer Celsius Fahrenheit Kelvin (absolute 0) Convert to Fahrenheit Convert to Celsius

Temperature In a qualitative manner, we can describe the temperature of an object as that which determines the sensation of warmth or coldness felt from contact with it. It is easy to demonstrate that when two objects are placed together (physicists say when they are put in thermal contact), the hotter object cools while the cooler object becomes warmer until a point is reached after which no more change occurs, and to our senses, they feel the same degree of warmth or coolness. When the thermal changes have stopped, we say that the two objects (physicists define them more rigorously as systems) are in thermal equilibrium . We can then define the temperature of the system by saying that the temperature is that quantity which is the same for both systems when they are in thermal equilibrium. If we experiment further with more than two systems, we find that many systems can be brought into thermal equilibrium with each other; thermal equilibrium does not depend on the kind of object used.

Put more precisely, if two systems are separately in thermal equilibrium with a third, then they must also be in thermal equilibrium with each other, and they all have the same temperature regardless of the kind of systems they are. The statement in italics, called the zeroth law of thermodynamics may be restated as follows: If three or more systems are in thermal contact with each other and all in equilibrium together, then any two taken separately are in equilibrium with one another. (quote from T. J. Quinn's monograph Temperature). Now one of the three systems could be an instrument calibrated to measure the temperature - i.e. a thermometer. When a calibrated thermometer is put in thermal contact with a system and reaches thermal equilibrium, we then have a quantitative measure of the temperature of the system. For example, a mercury-in-glass clinical thermometer is put under the tongue of a patient and allowed to reach thermal equilibrium in the patient's mouth - we then see by how much the silvery mercury has expanded in the stem and read the scale of the thermometer to find the patient's temperature.

Thermometer
A thermometer is an instrument that measures the temperature of a system in a quantitative way. The easiest way to do this is to find a substance having a property that changes in a regular way with its temperature. The most direct 'regular' way is a linear one: t(x) = ax + b
where t is the temperature of the substance and changes as the property x of the substance changes. The constants a and b depend on the substance used and may be evaluated by specifying two temperature points on the scale, such as 32° for the freezing point of water and 212° for its boiling point. For example, the element mercury is liquid in the temperature range of -38.9° C to 356.7° C (we'll discuss the Celsius ° C scale later). As a liquid, mercury expands as it gets warmer, its expansion rate is linear and can be accurately calibrated. The mercury-in-glass thermometer illustrated in the figure contains a bulb filled with mercury that is allowed to expand into a capillary. Its rate of expansion is calibrated on the glass scale.

Galileo Thermometer Based on a thermoscope invented by Galileo Galilei during the Renaissance in the early 1600s, the thermometer on the image at the right is called a Galileo thermometer. A simple, fairly accurate thermometer, today it is mostly used as decoration. The Galileo thermometer consists of a sealed glass tube that is filled with water and several floating bubbles. The bubbles are glass spheres filled with a colored liquid mixture. This liquid mixture may contain alcohol, or it might simply be water with food coloring. Attached to each bubble is a little metal tag that indicates a temperature. A number and degree symbol are engraved in the tag. These metal tags are actually calibrated counterweights. The weight of each tag is slightly different from the others. Since the bubbles are all hand-blown glass, they aren't exactly the same size and shape. The bubbles are calibrated by adding a certain amount of fluid to them so that they have the exact same density. So, after the weighted tags are attached to the bubbles, each differs very slightly in density (the ratio of mass to volume) from the other bubbles, and the density of all of them is very close to the density of the surrounding water. Because an object immersed in a fluid experiences two major forces: the downward pull of gravity and the upward push of buoyancy.

It is the downward force of gravity that makes this thermometer work. The basic idea is that as the temperature of the air outside the thermometer changes, so does the temperature of the water surrounding the bubbles. As the temperature of the water changes, it either expands or contracts, thereby changing its density. So, at any given density, some of the bubbles will float and others will sink. The bubble that sinks the most indicates the approximate current temperature.

Celsius
Anders Celsius, a Swedish astronomer, introduced his scale is 1742. For it, he used the freezing point of water as zero and the boiling point as 100. For a long time, the Celsius scale was called "centigrade." The Greek prefix "centi" means one-hundredth and each degree Celsius is one-hundredth of the way between the temperatures of freezing and boiling for water. The Celsius temperature scale is part of the "metric system" of measurement (SI) and is used almost throughout the world.

To convert Celsius temperatures into Fahrenheit:
Begin by multiplying the Celsius temperature by 9.
Divide the answer by 5.
Now add 32.
Here's an example: Change 20 degrees Celsius to Fahrenheit: 20 times 9 is 180. Then 180 divided by 5 is 36. Finally, 36 plus 32 is 68 degrees Fahrenheit.


Fahrenheit
On the Fahrenheit scale, the freezing point of water is 32 degrees and the boiling point is 212 degrees. Zero Fahrenheit was the coldest temperature that the German-born scientist Gabriel Daniel Fahrenheit could create with a mixture of ice and ordinary salt. He invented the mercury thermometer and introduced it and his scale in 1714 in Holland, where he lived most of his life.

To convert Fahrenheit temperatures into Celsius
Begin by subtracting 32 from the Fahrenheit number.
Divide the answer by 9.
Then multiply that answer by 5.

Here's an example: Change 95 degrees Fahrenheit to Celsius: 95 minus 32 is 63. Then, 63 divided by 9 is 7. Finally, 7 times 5 is 35 degrees Celsius. Time to go to the beach!

Kelvin, absolute zero Absolute Zero, lowest temperature theoretically possible, characterized by complete absence of heat. Absolute zero is -273.15° C (-459.67° F), or zero on the thermodynamic or Kelvin scale (0 K). The concept of an absolute zero of temperature first arose in connection with experiments with gases; when a fixed volume of gas is cooled, its pressure decreases with its temperature. Although this experiment cannot be conducted below the liquefaction point of the gas, a plot of the experimental values of pressure versus temperature can be extrapolated to zero pressure. The temperature at which the pressure would be zero is the absolute zero of temperature. This experimentally derived concept was subsequently shown to be consistent with theoretical definitions of absolute zero. The atoms and molecules in an object at absolute zero would have their minimum possible amount of motion. They would not be completely at rest, but they could not lose any more energy of movement, and so could not transfer any heat to another object. Absolute zero cannot actually be reached, although it can be arbitrarily closely approached. Special procedures are needed to reach very low, or cryogenic, temperatures. Liquid helium, which has a normal boiling point of 4.2 K (-268.9° C/-452.1° F), can be produced by cryostats, extremely well-insulated vessels based on a design by the American mechanical engineer Samuel Collins. If the helium is then evaporated at reduced pressures, temperatures as low as 0.7 K can be obtained. Lower temperatures require the successive magnetization and demagnetization of paramagnetic substances (substances of low magnetizability), such as chrome alum. The method, which was first developed in 1937 by the Canadian-American chemist William Giauque, utilizes a magnetic field that initially aligns the ionic magnets of the material, which is cooled in a liquid helium bath. If the magnetic field is removed, the magnets again assume their random orientation, reducing the thermal energy of the material, and thus its temperature, still further. Temperatures as low as 0.002 K have been reached with the demagnetization of paramagnetic salts, and the demagnetization of atomic nuclei has yielded temperatures as low as 0.00001 K. Temperature measurements at values close to absolute zero present special problems. Gas thermometers can only be used down to the liquefaction point of helium. At lower temperatures, electrical and magnetic measurements must be used to determine the effective temperature. The concept of absolute zero is also important in theoretical considerations. According to the third law of thermodynamics, the entropy, or state of disorder, of a pure crystal would be zero at absolute zero; this is of considerable importance in analysing chemical reactions and in quantum physics. Materials show strange properties when they are cooled to very low temperatures. Some lose their electrical resistance completely. This effect was first observed in mercury a few degrees above absolute zero, but it is being produced at ever higher temperatures in new materials.