It consists of a bulb connected by a capillary tube to amanometer. This device is called the constant volume gas thermometer, seeFigure 16.1). The standard thermometer, against which all other thermometers are to becalibrated, is based on the pressure exerted by a gas confined to a fixedvolume. Water can boil or freeze at differenttemperatures (depending on the pressure) and therefore boiling or freezing ofwater can not be used to define a standard temperature. The triple point of water hasbeen assigned a temperature of 273.16 K. Ice, water and vapor canonly coexist at one temperature and pressure. Thereference point is the triple point of water. The unit of temperature will be the Kelvin. The first step in defining a temperature scale is to pick out somereproducible thermal environment and, quit arbitrarily, assign a certaintemperature to it.
Thetemperature is a property of a body, and two bodies are found to be in thermalequilibrium if their temperatures are equal. This is called the zeroth law of thermodynamics. If the thermometer is also inthermal equilibrium with a second body than the two bodies are also in thermalequilibrium. After making contact, every measurableproperty of the thermometer and the body assumes a stable value, and the bodiesare said to be in thermal equilibrium. In order for a thermometer to measure the temperature of a body, it must be inintimate contact with the body. When the temperature increases, thevolume of a liquid increases, the length of a metal rod increases, theelectrical resistance increases, the pressure of a confined gas increases, etc.If we know the change in these parameters as a function of the temperature, wecan use them to measure the temperature. Properties of many bodies changeas their thermal environment is altered. The central concept ofthermodynamics is the temperature T. Thermodynamics deals with the internal energies of systems and is governed bya set of laws (similar to Newton's law for mechanics).
TEMPERATURE 16.TEMPERATURE 16.1.Thermal Equilibrium
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