TEMPERATURE

If there is any scientific concept familiar to everybody, it is the concept of temperature. All of our lives we have been poked with thermometers and listened to weather forecasts about tomorrow’s temperature. How is that quantity, measured by various kinds of thermometers, related to the atomic processes we have been discussing?
Most thermometers are a black art which depends upon such properties as the thermal expansion of mercury or alcohol, the stiffness of a spring, or the color changes of a material, etc. There is, however, one kind of a thermometer whose function can be understood from a simple molecular picture. That is the ideal gas thermometer which we will discuss shortly. We will see that for an ideal gas thermometer, the temperature reading is proportional to the average thermal kinetic energy of the gas molecules in the thermometer.
When you measure the temperature of an object, you have to wait until the thermometer and the object are in thermal equilibrium. (You wait until the reading on the thermometer in your mouth stops changing.) When in thermal equilibrium, the molecules of the object and those of the thermometer have the same average thermal kinetic energy. If we are using an ideal gas thermometer, the reading is proportional to this average kinetic energy. Thus if we use an ideal gas thermometer as an experimental definition of temperature, we are effectively defining temperature as being proportional to the average thermal kinetic energy of the molecules.