University of Virginia
Physics Department

## Thermometers

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.7

Objectives

Students will

• understand the mechanism through which thermometers record temperature;
• study various types of thermometers, including the mercury/alcohol thermometer, the Galileo thermometer and the bimetallic thermometer.

Motivation for Learning

Discrepant Event-Building an Air Thermometer

Materials

• Rubber stopper to fit bottle, with hole in center
• Rigid plastic tubing to fit hole in stopper, 24" long (glass tubing can be used, but it is dangerous)
• Water
• Food coloring
• Sealing wax or paraffin
• Scotch tape
• Cardboard strip, 10"x 2" (poster board or heavy paper will also work)
• Alcohol thermometer

Procedure

There are two possibilities to make a thermometer that are similar. The normal alcohol thermometers that we use contain alcohol with red coloring inside an enclosed glass tube. Because the glass tube is enclosed, atmospheric pressure does not affect the temperature measured. If the glass tube is open at the top, rather than closed, then atmospheric pressure can affect the height of the alcohol column thus affecting the reading. It is easier to build the later thermometer, but it is accurate only over short time periods when the atmospheric pressure is not changing.

1. Place the tubing through the hole in the stopper. Be very careful doing this if you are using glass tubing, because it is too easy to break the tubing and stick glass in your hand. Use cooking oil as lubricant to make it easier to put the tubing through the hole.
2. Color ½ pint water with dye or food coloring. Pour this into the glass bottle so that it is about ¼ full. It would be best if the water had already reached room temperature.
3. Place the stopper into the flask so that it makes a tight fit. The tube end should be in the liquid and almost touching the bottom. Use the colored water to fill the tube through the open end. Try to get the liquid to the bottom of the tube to replace the air. You may have to tilt the flask to do this.
4. Adjust the liquid level in the tube by loosening or tightening the stopper until the level is about halfway up the exposed portion of the tube above the stopper.
5. Use the wax to seal the tube to the stopper and the stopper to the bottle.
6. Cut three slits on the cardboard according to the diagram and slide it on the tube above the stopper.
7. If you used room temperature water in step 2, you may now mark the water level on the cardboard and note that this temperature is room temperature that you have measured with a regular thermometer. If you have not used room temperature water, you may have to wait a few hours until the thermometer has reached thermal equilibrium with its surroundings. Then record the air temperature with the thermometer. Mark the level of liquid on the cardboard as this temperature.
8. Place both the alcohol thermometer and your constructed thermometer in a new temperature; for example, in a tub of water at a much higher temperature. Make sure the water does not shoot out the top. If it does, you will need a longer tube.
9. Mark this new temperature on the cardboard.
10. If possible, repeat this a third time in a new temperature setting, for example in a refrigerator. Remove the cardboard from the glass tubing and measure the distances between the different temperature markings. From this, you can create an entire temperature scale; you have built an air thermometer!

Background Information

A thermometer is a device used to measure the temperature of a system. When a thermometer is in thermal equilibrium with the system, it measures its own temperature as well as that of the system. The most common thermometer utilizes a liquid, either mercury or alcohol. A glass bulb, filled with this liquid, is connected to a glass capillary tube. When heated, the liquid expands from the bulb into the capillary tube, so that the change in temperature corresponds with the change in height of the liquid. Mercury is commonly used because there is a wide temperature range at which it is liquid: from -38.9 °C to 356.7°C. More recently, alcohol thermometers have supplanted the use of mercury thermometers in schools because of dangers associated with mercury. If you use a mercury thermometer in the experiment, be especially careful that it does not break.

There are other, more complex types of thermometers as well. A Galileo thermometer applies the fact that liquids expand faster with increasing temperature than do solids. It is made of a column of fluid that contains solid spheres with a density close to that of the liquid. If the sphere's density is less than the liquid, it will float; if it is greater than the liquid, it will sink. As the temperature increases, the liquid will expand faster than the solid and the liquid's density will decrease at a quicker rate, so that the spheres will eventually sink. Each sphere in a Galileo thermometer is a specific density so that it will sink at an exact temperature, thus displaying the temperature of its surroundings. The photo at the left is of a Galileo thermometer, which are no longer commonly used.

A third type of thermometer, a bimetallic thermometer, is used in household applications such as the thermostat. This thermometer is made of two strips of different types of metal which are sandwiched together and rolled into a coil. The metals expand at different rates with an increase in temperature, causing the coil to either wind or unwind. A pointer connected to the bimetallic coil indicates the temperature.

Note that the thermostat may have a mercury bulb inside to act as a switch. MERCURY IS POISONOUS AND THE STUDENTS SHOULD NOT BE ALLOWED TO HANDLE THE THERMOSTAT. Click here for a description of the element mercury and how dangerous it can be. Most schools no longer use mercury thermometers for this reason, and your school system may not allow even a thermostat containing mercury inside the bulb to be in the school, although they probably use thermostats in the heating/cooling system that have mercury inside them. This activity is quite safe as long as the students are not allowed to handle the thermostat. If by some accident, the mercury bulb is broken, everyone should immediately move away. It should be safe to mount the thermostat on a board using screws. The board can be a convenient size, say 10" x 10", so that it will stay in one place. The thermostat needs to be in the vertical position. There are thermostats available that do not use mercury. One that we found is White-Rodgers brand Model 7200. This can be bought at a home store such as Lowe's.

### Student Activity

Materials

• Alcohol thermometer
• Ice bath (600 mL beaker with ice and water)
• Optional: thermostat (Honeywell tradeline T87F 2873). Go to a building supply company (like Lowe's) and purchase the cheapest (heat only) thermostat you find. It should cost about \$10. You can also purchase a heat/cool thermostat which will have two bimetallic strips inside, one for heat and one for cooling.

Procedure

1. Observe the temperature initially on the alcohol thermometer. Then place the alcohol thermometer in the ice bath. Observe and write down the indicated temperature every minute.
2. Observe how long it takes for the thermometer temperature to reach equilibrium.
3. Examine the bimetallic thermometer in the thermostat. If the thermostat has a mercury bulb inside, students should be very careful with the thermostat. The thermostat should be mounted vertically on a firm location like a board so students can observe it, but not handle it. Teachers should read the danger warning about mercury in the Background Information. Put the thermostat in a cool area (refrigerator, in front of an air conditioner vent, or blow air over a container of ice) and observe what happens. Place it in a warm area and observe what happens. Blowing hot and cold air from a hair dyer may be a good option.

Data Sheet

Questions

1. How long did it take for the thermometer to record the equilibrium temperature? What happened in the time it took for the reading to be accurate?

2. How can the air thermometer accurately record temperature? Describe its aspects that are similar to those of an alcohol thermometer.

3. Why do liquids expand at a faster rate than solids? How does this help a Galileo thermometer record temperature?

Students with Special Needs

All students should be able to participate in this activity.

Click here for further information on laboratories with students with special needs.

Assessment

Fill out data sheet during activity.