| | Water is poured into two compartments of a container separated by a thin metal wall (Fig. 1, Movie 1). In one of the compartments there is hot water, in the other one the water is cold. The container is well insulated and covered by a styrofoam lid. The bodies of water are stirred continuously, so there is a uniform temperature for each body. It is found that the temperature of the hot water falls whereas the temperature of the cold water rises. This goes on until both temperatures are the same (Fig. 1, left diagram). Temperatures equilibrate for different bodies as well (a block of cold copper in hot water, Fig. 1, right diagram; see Movie 2)). This shows that temperatures become equal, not quantities of heat. (*)
Interpretation For a body of water to cool down, it has to lose heat, for it to get warmer, it must gain heat. We say that heat flows from the hotter body of water to the colder one as long as there is a temperature difference. Since the behavior of the temperatures resembles that of water levels in communicating tanks, or of voltages of capacitors connected by a resistor, we interpret temperature as the thermal level quantity. The water in the container would lose heat to the cool environment, and the temperatures would drop further were it not for the insulation. Styrofoam insulation makes it difficult for heat to flow out of warm bodies.
 (*) Note: The technical term for quantities of heat is entropy. What we call heat, and what is officially called entropy by engineers and physicsist is closely related to what scientisits before 1850 called caloric. When presenting phenomena in terms of standard language, it makes sense to keep using the word heat for what we normally call entropy.
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