Heat - previous index next Heat Michael Fowler University...

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previous index next Heat Michael Fowler, University of Virginia Feeling and seeing temperature changes Within some reasonable temperature range, we can get a rough idea how warm something is by touching it. But this can be unreliable—if you put one hand in cold water, one in hot, then plunge both of them into lukewarm water, one hand will tell you it’s hot, the other will feel cold. For something too hot to touch, we can often get an impression of how hot it is by approaching and sensing the radiant heat. If the temperature increases enough, it begins to glow and we can see it’s hot! The problem with these subjective perceptions of heat is that they may not be the same for everybody. If our two hands can’t agree on whether water is warm or cold, how likely is it that a group of people can set a uniform standard? We need to construct a device of some kind that responds to temperature in a simple, measurable way—we need a thermometer. The first step on the road to a thermometer was taken by one Philo of Byzantium , an engineer, in the second century BC. He took a hollow lead sphere connected with a tight seal to one end of a pipe, the other end of the pipe being under water in another vessel. To quote Philo: “…if you expose the sphere to the sun, part of the air enclosed in the tube will pass out when the sphere becomes hot. This will be evident because the air will descend from the tube into the water, agitating it and producing a succession of bubbles . Now if the sphere is put back in the shade, that is, where the sun’s rays do not reach it, the water will rise and pass through the tube …” No matter how many times you repeat the operation, the same thing will happen . In fact, if you heat the sphere with fire, or even if you pour hot water over it, the result will be the same .”
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2 Notice that Philo did what a real investigative scientist should do—he checked that the experiment was reproducible , and he established that the air’s expansion was in response to heat being applied to the sphere, and was independent of the source of the heat . Classic Dramatic Uses of Temperature-Dependent Effects This expansion of air on heating became widely known in classical times, and was used in various dramatic devices. For example, Hero of Alexandria describes a small temple where a fire on the altar causes the doors to open . The altar is a large airtight box, with a pipe leading from it to another enclosed container filled with water. When the fire is set on top of the altar, the air in the box heats up and expands into a second container which is filled with water. This water is forced out through an overflow pipe into a bucket hung on a rope attached to the door hinges in such a way that as the bucket fills with water, it drops, turns the hinges, and opens the doors. The pipe into this bucket reaches almost to the bottom, so that when the altar fire goes out, the water is sucked back and the doors close again. (Presumably, once the fire is burning, the god behind the doors is ready to do business and the doors open…)
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This note was uploaded on 12/07/2011 for the course PHYSICS 152 taught by Professor Michaelfowler during the Fall '07 term at UVA.

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Heat - previous index next Heat Michael Fowler University...

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