Thermometers: Old and New
Temperature is how hot or cold something is as indicated by a particular scale of measurement. Your body is sensitive to degrees of hotness or coldness of an object but not to specific temperatures. From as early as 1593, thermometers have been used to measure temperature.
In this project, you will study the progression of measuring temperature from the earliest open-air thermometer designed by the Italian physicist Galileo Galilei called a "thermoscope" to modern thermometers that use "chameleon" chemicals to indicate temperature. You will also look at the algebraic relationships between the Fahrenheit, Celsius, and Kelvin temperature scales.
Purpose: To construct a model of Galileo's gas thermometer.
- l-quart (I-liter) jar
- blue food coloring
- drinking straw
- glass soda bottle
- modeling clay
- Pour about 2 inches (5 cm) of water into the jar.
- Add drops of blue food coloring to make the water in the jar a deep blue.
- Insert about 2 inches (5 cm) of the end of the straw into the mouth of the soda bottle.
- Mold a piece of clay around the straw and seal the mouth of the bottle.
- Wrap your hands around the sides of the bottle. Press as much of the palms of your hands as possible against the glass, but do not press hard enough to break the glass.
- Hold the bottle in your hands for one minute (see Figure 29.1).
- Stand the bottle, straw down, in the jar of colored water so that the straw extends below the surface of the water (see Figure 29.2).
- Observe the straw for two to three minutes.
The colored water rises in the straw.
A thermometer is used to measure the average energy of motion of molecules. The bottle is a simplified model of Galileo's gas thermometer. Galileo designed his thermometer, which he called a "thermoscope," in 1593. The thermoscope and the bottle thermometer both make use of the fact that gas expands when heated and contracts when cooled to indicate a change in temperature. The bottle and straw are filled with air. Holding the bottle in your hands causes the gas inside to be heated. The heated gas molecules expand (move faster and farther apart) and escape through the end of the straw. As the bottle cools, the gas molecules that are left in the bottle contract (move slower and closer together). Since there are now fewer molecules in the bottle, they take up less space and a partial vacuum (an empty space) is created. The pressure inside the bottle is less than the atmospheric pressure outside; thus, air pushing on the surface of the water forces water into the straw.
Try New Approaches
- Does adding more energy affect the results? Repeat the experiment standing the bottle in a bowl containing 2 inches (5 cm) of hot tap water. Mark the height of the water column in the straw or its level in the bottle. Use a thermometer to measure the temperature in degrees Fahrenheit or Celsius.
- Do thermometers measure the temperature or heat content of a substance? Temperature is the measure of the kinetic energy (energy of motion) of the molecules, and heat is the combined energy of all the molecules in the substance. Demonstrate the difference between the temperature and heat content by filling a large baking pan with 2 inches (5 cm) of warm water at the same temperature as that used in the previous experiment. Set the bottle thermometer in the bowl of water and observe the height of the water column in the straw or inside the bottle. Note: It is important to use the same height of water so that the same number of gas molecules inside the bottle are heated. Science Fair Hint: Display photographs of the thermoscope model.
- Does cooling the bottle affect the results? Repeat the original experiment setting the bottle in a bowl of ice water. Science Fair Hint: Display diagrams of the thermoscope model showing the movement of air in and out of the straw as the bottle is heated and cooled.