- University of Virginia
- Physics Department
How Does a Thermostat Work?
A Physical Science Activity
2003 Virginia SOLs
- understand the various parts of a thermostat, including the bimetallic thermometer
and the mercury-filled bulb;
- visualize the effect of hot and cold air on the thermostat;
- understand how a thermostat is connected to a circuit that regulates the
heat produced in a house.
Discrepant Event-The Thermostat Circuit
- Honeywell Thermostat (Model T87F 2873 for heating only) or other simple
household thermostat containing a mercury filled tube and bimetallic thermometer
- Light bulb (#40, rated for 6.3 volts)
- Light bulb holder
- 4 D cell Batteries
- Battery holder for 4 batteries (available from Radio Shack)
- Hair dryer with both hot and cold air options
- 2 wires with alligator clips
- Desk Lamp (Heat)
- Refrigerator (Cold)
- 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.
- Create a simple series circuit between the thermostat wire end points,
the light bulb and the batteries.
- 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
- 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.
- 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
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
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.
To print out the Student Copy only, click
- Honeywell Thermostat (Model number T87F 2873)
- Hair dryer with both heating and cooling options, or other heating/cooling
methods (SEE DISCREPANT EVENT)
- 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.
- Locate the bimetallic coil and the mercury bulb. Sketch a diagram of the
thermostat in your data table, labeling any important parts.
- Plug in the hair dryer. Blow hot air on the bimetallic coil. Observe what
happens and record your observations.
- Switch the hair dryer so that it blows cold (or room temperature) air.
Observe any changes in the thermostat structure and record these.
- 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.
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:
- Explain how a bimetallic coil reacts to a change in temperature.
- Explain in your own words how a change in air temperature impacts
the function of a thermostat.
- 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.
Data sheet to be completed during the laboratory.