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Temperature is one of the fundamental concepts of physics. We are all able to recognize that some bodies are hotter than others, but our temperature sense is qualitative rather than quantitative and is capable of only a limited range. The sense of touch can frequently be used to distinguish between hotter and colder objects, provided that these lie in a temperature range consistent with the stability of human tissue. Even within this range the sense of touch is often unreliable as a measure of temperature.
The metal bracket holding a wooden rail may feel much colder to the touch than the railing itself, even though both are at the same temperature. It is a matter of common observation that some of the physical properties of many substances are altered when the temperature is changed. The volume or the pressure of a gas increases when the temperature is raised. The length of a copper rod changes with changing temperature. Some of the electric and magnetic properties of substances vary with changes in temperature. The changes that take place in these physical properties can be used to measure the changes in temperature which produced them.