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A flat plastic thermometer used to measure forehead temperature; the thermometer can measure between ninety-five and one-hundred four degrees Fahrenheit, or between thirty-five and forty degrees Celsius.
Each of the six squares on this plastic (liquid crystal) thermometer contains a film of a different heat-sensitive liquid crystal material. Below 95 º F size 12{"95"°F} {} , all six squares are black. When the plastic thermometer is exposed to temperature that increases to 95 º F size 12{"95"°F} {} , the first liquid crystal square changes color. When the temperature increases above 96 . 8 º F size 12{"96" "." 8°F} {} the second liquid crystal square also changes color, and so forth. (credit: Arkrishna, Wikimedia Commons)
A man holds a device that looks like a gun or a check-out scanner up toward an air vent. A red light emanates from the device and shines on the vent.
Fireman Jason Ormand uses a pyrometer to check the temperature of an aircraft carrier’s ventilation system. Infrared radiation (whose emission varies with temperature) from the vent is measured and a temperature readout is quickly produced. Infrared measurements are also frequently used as a measure of body temperature. These modern thermometers, placed in the ear canal, are more accurate than alcohol thermometers placed under the tongue or in the armpit. (credit: Lamel J. Hinton/U.S. Navy)

Temperature scales

Thermometers are used to measure temperature according to well-defined scales of measurement, which use pre-defined reference points to help compare quantities. The three most common temperature scales are the Fahrenheit, Celsius, and Kelvin scales. A temperature scale can be created by identifying two easily reproducible temperatures. The freezing and boiling temperatures of water at standard atmospheric pressure are commonly used.

The Celsius scale (which replaced the slightly different centigrade scale) has the freezing point of water at 0 º C size 12{0°C} {} and the boiling point at 100 º C size 12{"100"°C} {} . Its unit is the degree Celsius     ( º C ) size 12{ \( °C \) } {} . On the Fahrenheit scale (still the most frequently used in the United States), the freezing point of water is at 32 º F size 12{"32"°F} {} and the boiling point is at 212 º F size 12{"212"°F} {} . The unit of temperature on this scale is the degree Fahrenheit     ( º F ) size 12{ \( °F \) } {} . Note that a temperature difference of one degree Celsius is greater than a temperature difference of one degree Fahrenheit. Only 100 Celsius degrees span the same range as 180 Fahrenheit degrees, thus one degree on the Celsius scale is 1.8 times larger than one degree on the Fahrenheit scale 180 / 100 = 9 / 5 . size 12{"180"/"100"=9/5 "." } {}

The Kelvin scale is the temperature scale that is commonly used in science. It is an absolute temperature scale defined to have 0 K at the lowest possible temperature, called absolute zero    . The official temperature unit on this scale is the kelvin , which is abbreviated K, and is not accompanied by a degree sign. The freezing and boiling points of water are 273.15 K and 373.15 K, respectively. Thus, the magnitude of temperature differences is the same in units of kelvins and degrees Celsius. Unlike other temperature scales, the Kelvin scale is an absolute scale. It is used extensively in scientific work because a number of physical quantities, such as the volume of an ideal gas, are directly related to absolute temperature. The kelvin is the SI unit used in scientific work.

Three temperature scales—Fahrenheit, Celsius, and Kelvin—are oriented horizontally, one below the other, and aligned to show how they relate to each other. Absolute zero is at negative four hundred fifty nine point six seven degrees F, negative two hundred seventy three point one five degrees C, and 0 K. Water freezes at thirty two degrees F, 0 degrees C, and two hundred seventy three point one five K. Water boils at two hundred twelve degrees F, one hundred degrees C, and three hundred seventy three point one five K. A temperature difference of 9 degrees F is the same as a temperature difference of 5 degrees C and 5 K.
Relationships between the Fahrenheit, Celsius, and Kelvin temperature scales, rounded to the nearest degree. The relative sizes of the scales are also shown.
Practice Key Terms 9

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Source:  OpenStax, College physics (engineering physics 2, tuas). OpenStax CNX. May 08, 2014 Download for free at http://legacy.cnx.org/content/col11649/1.2
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