Thermometers and temperature scales

The sense of touch provides some indication of the temperature of an object but is unreliable. For example, the metal shelf in the refrigerator feels colder than the food sitting on the shelf, even though they are in thermal equilibrium. The metal feels colder because the metal conducts the heat from your hand more efficiently.
Thermometers are instruments that define and measure the temperature of a system. The common thermometer consists of a volume of mercury that expands into a capillary tube when heated. When the thermometer is in thermal equilibrium with an object, the temperature can be read from the thermometer scale.

Three temperature scales are commonly used: Celsius, Fahrenheit, and Kelvin (also called absolute). Comparisons of the Celsius and Fahrenheit thermometers are shown in Figure 1 .

Figure:  Comparison of Celsius and Fahrenheit thermometers. Figure:  Comparison of Celsius and Fahrenheit thermometers.

On the Celsius scale, the ice point is 0, and the steam point is 100. The interval between these temperatures is divided into 100 equal parts called degrees. As shown in Figure 1 , on the Fahrenheit scale, the ice point is 32 degrees, and the steam point is 212 degrees. The interval between these temperatures is divided into 180 equal parts. The following equations relate temperature in Celsius (C) and Fahrenheit (F):

The Kelvin scale (K) has degrees of the same size as the Celsius scale, but the zero is shifted to the triple point of water. The triple point of water exists when water within a closed vessel is in equilibrium in all three states: ice, water, and vapor. This point is defined as 273.16 Kelvin and equals .01 degrees Celsius; therefore, to convert Celsius to Kelvin, simply add 273.15. Note that because the degrees are the same in the two scales, temperature differences are the same in either Celsius or Kelvin.