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Name: Karen
Status: educator
Grade: 4-5
Location: FL 
Country: N/A
Date: 2/7/2005


Question:
How does a Galileo Thermometer work?


Replies:
Karen,

The Galileo thermometer consists of a sealed glass tube that is filled with water and several floating bubbles. The bubbles are glass spheres filled with a colored liquid mixture. This liquid mixture may contain alcohol, or it might simply be water with food coloring. Attached to each bubble is a little metal tag that indicates a temperature. A number and degree symbol are engraved in the tag. These metal tags are actually calibrated counterweights. The weight of each tag is slightly different from the others. Since the bubbles are all hand-blown glass, they are not exactly the same size and shape. The bubbles are calibrated by adding a certain amount of fluid to them so that they have exact same density. So, after the weighted tags are attached to the bubbles, each differs very slightly in density from the other bubbles, and the density of all of them is very close to the density of the surrounding water.

The basic idea is that as the temperature of the air outside the thermometer changes, so does the temperature of the water surrounding the bubbles. As the temperature of the water changes, it either expands or contracts, thereby changing its density. At this changed density, some of the bubbles will float and others will sink. The above has been excerpted from How Stuff Works.com. I hope that this helps.

Sincerely,

Bob Trach


Based on a thermoscope invented by Galileo Galilei in the early 1600s, the thermometer on your co-worker's desk is called a Galileo thermometer. A simple, fairly accurate thermometer, today it is mostly used as decoration. The Galileo thermometer consists of a sealed glass tube that is filled with water and several floating bubbles. The bubbles are glass spheres filled with a colored liquid mixture. This liquid mixture may contain alcohol, or it might simply be water with food coloring.

Attached to each bubble is a little metal tag that indicates a temperature. A number and degree symbol are engraved in the tag. These metal tags are actually calibrated counterweights. The weight of each tag is slightly different from the others. Since the bubbles are all hand-blown glass, they are not exactly the same size and shape. The bubbles are calibrated by adding a certain amount of fluid to them so that they have the exact same density. So, after the weighted tags are attached to the bubbles, each differs very slightly in density (the ratio of mass to volume) from the other bubbles, and the density of all of them is very close to the density of the surrounding water.

If you have read this question, then you know that an object immersed in a fluid experiences two major forces: the downward pull due to gravity and the upward push of buoyancy. It is the downward force due to gravity that makes this thermometer work.

The basic idea is that as the temperature of the air outside the thermometer changes, so does the temperature of the water surrounding the bubbles. As the temperature of the water changes, it either expands or contracts, thereby changing its density. So, at any given density, some of the bubbles will float and others will sink. The bubble that sinks the most indicates the approximate current temperature.

Consider this example:

Let us say there are five bubbles in the thermometer:

* A blue bubble that represents 60 degrees

* A yellow bubble that represents 65 degrees

* A green bubble that represents 70 degrees

* A purple bubble that represents 75 degrees

* A red bubble that represents 80 degrees

The blue bubble (60 degrees) is the heaviest (densest) bubble, and each bubble thereafter is slightly lighter, with the red bubble being the lightest. Now, let us say the temperature in the room is 70 degrees. Since the surrounding air is 70 degrees, we know the water inside the thermometer is also about 70 degrees. The blue and yellow bubbles (60 and 65 degrees, respectively) are calibrated so that they have higher densities than the water at this temperature, so they sink. The purple and red bubbles each have a density that is lower than the surrounding water, so they float at the very top of the thermometer. Since the green bubble is calibrated to represent 70 degrees, the same temperature as the water, it sinks slightly so that it is floating just below the purple and red bubbles -- thereby indicating the room's temperature!

Here are some interesting links:

* How do ear thermometers work?

http://www.howstuffworks.com/question663.htm

James Przewoznik


A Galileo thermometer uses some sealed glass balls or bubbles that are floating (or almost floating) are in a bath of water. When the temperature increases, the bubbles sink one by one, and this indicates the temperature

Here is how it works. The weights of the bubbles are precisely set using both small masses hanging from the bottom, and also putting some colored liquid into the bubble before sealing it. Each bubble is made so that it is very close to the density of the water. An object immersed in a fluid experiences and upward force (the buoyant force equal to the weight of water displaced) and a downward force (equal to the weight of the object.)

When the temperature goes up, the size of the glass bubbles hardly changes because the thermal expansion coefficient of glass is small. But the water becomes less dense because it expands. When the density decreases, the glass bubble feels less of a buoyant force and it sinks. At the highest temperature, the lightest bubble finally sinks.

A web site www.howstuffworks.com has a good explanation .

Bob Erck



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