12.3

Thermometers

All thermometers make use of the change in some physical property with temperature. A property that changes with temperature is called a thermometric property. For example, the thermometric property of the mercury thermometer is the length of the mercury column, while in the constant-volume gas thermometer it is the pressure of the gas. Several other important thermometers and their thermometric properties will now be discussed.
The thermocouple is a thermometer used extensively in scientific laboratories. It consists of thin wires of different metals, welded together at the ends to form two junctions, as Figure 12-5 illustrates. Often the metals are copper and constantan (a copper–nickel alloy). One of the junctions, called the “hot” junction, is placed in thermal contact with the object whose temperature is being measured. The other junction, termed the “reference” junction, is kept at a known constant temperature (usually an ice–water mixture at 0 °C). The thermocouple generates a voltage that depends on the difference in temperature between the two junctions. This voltage is the thermometric property and is measured by a voltmeter, as the drawing indicates. With the aid of calibration tables, the temperature of the hot junction can be obtained from the voltage. Thermocouples are used to measure temperatures as high as 2300 °C or as low as −270 °C.

Figure 12-5 (a) A thermocouple is made from two different types of wires, copper and constantan in this case. (b) A thermocouple junction between two different wires. (Courtesy Omega Engineering, Inc.)

Most substances offer resistance to the flow of electricity, and this resistance changes with temperature. As a result, electrical resistance provides another thermometric property. Electrical resistance thermometers are often made from platinum wire, because platinum has excellent mechanical and electrical properties in the temperature range from −270 °C to +700 °C. The resistance of platinum wire is known as a function of temperature. Thus, the temperature of a substance can be determined by placing the resistance thermometer in thermal contact with the substance and measuring the resistance of the wire.
Radiation emitted by an object can also be used to indicate temperature. At low to moderate temperatures, the predominant radiation emitted is infrared. As the temperature is raised, the intensity of the radiation increases substantially. In one interesting application, an infrared camera registers the intensity of the infrared radiation produced at different locations on the human body. The camera is connected to a color monitor that displays the different infrared intensities as different colors. This “thermal painting” is called a thermograph or thermogram. Thermography is an important diagnostic tool in medicine. For example, breast cancer is indicated in the thermograph in Figure 12-6 by the elevated temperatures associated with malignant tissue. In another application, Figure 12-7 shows thermographic images of a smoker’s forearms before (left) and 5 minutes after (right) he has smoked a cigarette. After smoking, the forearms are cooler due to the effect of nicotine, which causes vasoconstriction (narrowing of the blood vessels) and reduces blood flow, a result that can lead to a higher risk from blood clotting. Temperatures in these images range from over 34 °C to about 28 °C and are indicated in decreasing order by the colors white, red, yellow, green, and blue.

Figure 12-6 Invasive carcinoma (cancer) of the breast registers colors from red to yellow/white in this thermograph, indicating markedly elevated temperatures. (Science Photo Library/Photo Researchers, Inc.)

Figure 12-7 Thermogram showing a smoker’s forearms before (left) and 5 minutes after (right) he has smoked a cigarette. Temperatures range from over 34 °C (white) to about 28 °C (blue). (Dr. Arthur Tucker/Science Photo Library/Photo Researchers, Inc.)

The physics of thermography.

Oceanographers and meteorologists also use thermographs extensively, to map the temperature distribution on the surface of the earth. For example, Figure 12-8 shows a satellite image of the sea-surface temperature of the Pacific Ocean. The region depicted in red is the 1997/98 El Niño, a large area of the ocean, approximately twice the width of the United States, where temperatures reached abnormally high values. This El Niño caused major weather changes in certain regions of the earth.

Figure 12-8 A thermogram of the 1997/98 El Niño (red), a large region of abnormally high temperatures in the Pacific Ocean. (Courtesy NOAA)

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