University of Virginia
Physics Department

How Does a Thermostat Work?

A Physical Science Activity

2003 Virginia SOLs

 

Objectives

Students will

Motivation for Learning

Discrepant Event-The Thermostat Circuit

Materials

Procedure

  1. Remove the cover from the thermostat. Locate the mercury-filled glass tube and identify the wires that are connected to the tube. Follow the wires and trace them to their endpoints (two small screws). There may be more than two wires. If not clear which two wires to use, hook up the ohmmeter part of a multimeter to the ends of the wires in question and check for the resistance when the mercury bulb is rotated both ways (change the temperature setting on the thermostat to rotate the bulb). When the mercury is at the end of the bulb where the wires connect, you should read zero resistance (short circuit). When the mercury is at the other end, you should read infinite resistance (open circuit). The thermostat must always remain in the vertical position as shown in order for the mercury bulb to function properly.

  2. Create a simple series circuit between the thermostat wire end points, the light bulb and the batteries.

  3. Explain to the class how the light bulb circuit is similar to the circuit within a home. Alternate the temperature settings on the thermostat and observe the circuit as the light bulb goes on and off. In your house the light bulb going on and off is replaced by your heating system going on and off. Tell the students that many thermostats have two mercury bulbs - one for heating and one for cooling. We are only dealing with a heating thermostat in this activity, but they may want to look at the one at home. Many modern thermostats are all solid state and do not have bimetallic strips and mercury bulbs.

  4. Set the temperature at 75 or 80°, so that the light bulb has just turned on. Using the hair dryer or desk lamp, apply heat to the bimetallic coil until the bimetallic coil heats up, expands and rotates, and causes the mercury bulb to rotate enough to disconnect the circuit. You should observe the mercury flowing from one end of the tube to the other. The circuit is disconnected, causing the light bulb to turn off. The effect would be to turn off the furnace.

  5. Set the thermostat so the light has just turned off (same as the furnace being off). Blow cool air onto the bimetallic coil using the hair dryer, or place the circuit in a refrigerator/ice box. If the hair dryer does not have the option of cool air, a solution would be to blow air over a container of ice, so that the ice cools the air reaching the bimetallic coil (see figure). Eventually, the circuit will be turned on and the light bulb will glow indicating that the temperature has dropped enough so that the heating furnace must turn back on.

 

Background Information

MERCURY IS POISONOUS AND THE STUDENTS SHOULD NOT BE ALLOWED TO HANDLE THE THERMOSTAT. See http://www.ch.adfa.oz.au/ozchemnet/web-elements/Hg.html#key 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. 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.

A thermostat's main purpose is to turn on the furnace in a house or apartment whenever the temperature falls below a certain preset level, and to turn off the furnace when the temperature rises above a set level. There is usually a few degrees of temperature difference between the minimum and maximum temperatures, so that the furnace doesn't repeatedly turn on and off. A simple home thermostat has two main parts: a bimetallic thermometer and a mercury-filled glass bulb. The bimetallic thermometer is made of two different metals that are sandwiched together and rolled into a coil.

A change in temperature causes the two metals to expand at different rates, and the coil therefore winds or unwinds. The bimetallic strip (or coil) is located adjacent to the glass bulb, so that as the coil expands or contracts it tips the glass bulb. The mercury falls from one end of the bulb to the other, which allows an electric current to flow between two wires or not flow. This current causes the furnace to turn on or off. If the temperature increases, the bimetallic coil changes shape and allows the mercury to fall to the other end of the bulb. When this happens, the current is unable to flow and the furnace turns off.

Usually, when the air temperature is the same as the temperature set on the thermostat, the mercury in the bulb is on one side. If the knob is turned to change the temperature, the bulb tilts to allow the air temperature to adjust. Once the temperature is correct again, the bulb readjusts the mercury level.


In this experiment, the students will examine a simple home thermostat. A suggested model is the Honeywell Tradeline (T87F 2873), which is a round, low-voltage thermostat. If the cover is removed, the bimetallic coil and mercury-filled bulb are both visible (see photo), although wires and a metal clip obscure the mercury bulb somewhat. Allow the students to observe the circuit that you have constructed. They should have the opportunity to examine its individual parts, but remember that they are not allowed to handle the thermostat. You should be the one to put the thermostat in different temperatures and observe the response of the bimetallic coil while they observe at a close range.

Note that all thermostats also have a thermometer on them in order to know what the current temperature is. In the model shown, the thermometer is on the bottom. This thermometer is also normally made of a bimetallic coil. In this thermostat it is too difficult to take off the front cover to see the second bimetallic coil for the thermometer.


Student Activity

To print out the Student Copy only, click here.

Materials

Procedure

  1. The thermostat must already be mounted vertically on a board or backing that is firmly placed. The students should not handle the thermostat, but they should carefully examine it.
  2. Locate the bimetallic coil and the mercury bulb. Sketch a diagram of the thermostat in your data table, labeling any important parts.
  3. Plug in the hair dryer. Blow hot air on the bimetallic coil. Observe what happens and record your observations.
  4. Switch the hair dryer so that it blows cold (or room temperature) air. Observe any changes in the thermostat structure and record these.
  5. Alternate the temperature of air on the thermostat. Observe the time it takes for the change in temperature to affect the position of the glass bulb.

Data Sheet

To print out the Data Sheet only, click here.

Diagram of Thermostat:

 

 

  

Observations after applying hot air:

 

 

  

Observations after applying cold air:

 

 

  

Observations after alternating temperatures:

 

 

  
  1. Explain how a bimetallic coil reacts to a change in temperature.


  2. Explain in your own words how a change in air temperature impacts the function of a thermostat.


  3. Name an application of thermostats other than to regulate the temperature of your house


Students with Special Needs

All students should be able to participate in the activity.

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


 

Assessment

Data sheet to be completed during the laboratory.