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Two bodies have equality of temperature if, when they are in thermal communication, no change in any observable property occurs. The title may also be read as Equilibrium of Temperature.
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Although temperature is a familiar property, defining it exactly is difficult. Temperature may be understood in terms of a sense of hotness or coldness when touching an object. When a hot body and a cold body are brought into contact, the hot body becomes cooler and the cold body becomes warmer. If these bodies remain in contact for some time, they usually appear to have the same hotness or coldness. However, one's sense of hotness or coldness is very unreliable. Sometimes very cold bodies may seem hot, and bodies of different materials that are at the same temperature appear to be at different temperatures.
Because of these difficulties in defining temperature, the equality of temperature is defined. Consider two blocks of copper, one hot and the other cold, each of which is in contact with a mercury-in-glass thermometer. If these two blocks of copper are brought into thermal communication, it is observed that the electrical resistance of the hot block decreases with time and that of the cold block increases with time. After a period of time has elapsed, however, no further changes in resistance are observed. Similarly, when the blocks are first brought in thermal communication, the length of a side of the hot block decreases with time, but the length of a side of the cold block increases with time. After a period of time, no further change in length of either of the blocks is perceived. In addition, the mercury column of the the thermometer in the hot block drops at first and that in the cold block rises, but after a period of time no further changes in height are observed. It may then be said, therefore, that two bodies have equality of temperature if, when they are in thermal communication, no change in any observable property occurs.
Related
▪ L - Zeroth Law of Thermodynamics