|MadSci Network: Physics|
Thanks for the question. After reading it carefully I believe there are a couple of ways to interpret the request. I will try to explore a few of them. There are a lot of different types of thermometers as I'm sure you know. The most common type of thermometer is the clinical thermometer; it is also sometimes referred to as a bulb thermometer. Your question asks how and why the temperature sample might differ by changing the bulb size. We should first accept the fact that a clinical thermometer does not measure temperature just because it has a bulb filled with mercury, or more commonly, alcohol. The device itself is carefully designed to measure the expansion of a certain amount of expandable fluid versus a linear distance through the thermometer cavity. Simply adding a larger bulb without changing the other properties of the thermometer will certainly give you a different result. Of course, not all thermometers have the same size bulb, but each thermometer is designed to measure a certain amount of fluid expanding over an expected distance so that each thermometer returns the correct result. Here are some possible scenarios: Larger bulb (with more Mercury or Alcohol) added to a thermometer designed for a small bulb: This will likely give a result of more heat, because more fluid will expand through the capillary tube. Larger Bulb (with same amount of Mercury or Alcohol) added to a thermometers designed for a small bulb: The fluid will have to expand into the empty cavity of the partially filled bulb and then expand into the capillary tube, this may give varying results depending on how much of the bulb is empty and free for expansion. Larger Bulb (Solid glass except for the original bulb cavity and fluid in the center) for said thermometer design: The glass around any thermometer fluid must expand first before any fluid inside can expand. This is often the cause of some thermometers dropping in temperature readings at first and then rising to meet the conditions of the test. More glass would take longer to expand and may give you a slightly lower temperature reading. Larger Bulb on a thermometer designed for a Larger Bulb: This will probably give you the correct temperature. Some things to be aware of: When you test temperature through this type of heat transfer, there will always be losses. Some of the heat you are measuring is stolen by the thermometer to expand the fluid. The equilibrium temperature is the result. Larger bulb thermometers will require more energy and change the conditions of the test more than small bulb thermometers. (Clinical thermometers are not extremely accurate - good for telling the temperature of a human or animal and designed exactly for such.) Also, Mercury and Alcohol have something in common, that being they both expand to an expected linear distance for the thermometer type. Mercury and alcohol differ dramatically in other ways. For one, Mercury has an extremely high level of fluid conductivity whereas alcohol has a very low fluid conductivity. *The conductivity of a material to electricity is related to the conductivity of heat and mercury is metal. That being said, putting mercury in a thermometer tube designed for alcohol will give very different results. Mercury also expands faster, in fact mercury thermometers have been known to explode due to the force of the expansion inside. * The conductivity relationship does not work at extremely high temperatures. Here are some websites related to this discussion. Although there doesn't seem to be a lot of direct answers on the web to your question, which makes it kind of unique! Heat transfer: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html#c2 Thermometers: http://www.coolquiz.com/trivia/explain/docs/thermometer.asp Temperature: http://eo.ucar.edu/skymath/tmp2.html I hope I helped you find your answer. -Dan
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