The Bi-metallic Thermostat
Thermostat—A device to maintain the room temperature at a certain minimum or maximum
temperature.
Bi-metallic—composed of two different metals that are bonded together
Thermal Expansion—when a material expands when heated (very few materials contract when
heated, but there are some)
Materials:
Bi-metallic strip, cork, two nails, matches
Optional
Alligator clips, battery, light bulb
1. Make a slit about 1 cm in the end of the cork (not through the center).
2. Nail the cork through the center to the base board.
3. Nail the second nail in the baseboard close enough so the bimetallic strip
can touch it.
4. Place the bi-metallic strip in the slit in the cork and let it touch the second
nail.
5. (optional) Connect the alligator clip to the bimetallic strip and the battery.
Connect another alligator clip from the battery to the light bulb. Connect a
third alligator clip from the light bulb to the second nail in the base board.
6. When the bimetallic strip touches the nail, it completes the electrical circuit
and turns on the furnace to warm the house (turns on the light bulb). Light a
match and place it under the bi-metallic strip to simulate the house warming
up.
7. When the bimetallic strip bends away from the nail, it breaks electrical
contact and turns the furnace off (remove the match from under the bimetallic strip).
8. As the bi-metallic strip cools, it will bend back to the way it was before,
touch the nail, and again start the furnace (light the match again).
9. You can rotate the cork which causes the bi-metallic strip to move. This is
like setting the temperature at which you want the room to stay.
10. You can use this thermostat to set the temperature for air-conditioning or
cooling. You need to reverse the direction the strip bends by keeping the
same end in the cork, just turn it up-side-down. Then it will bend back and
touch the nail as it warms and the thermostat turns on the air conditioner
which cools the bi-metallic strip and as it cools, it bends away from the
second nail and breaks contact, turning off the air conditioner. Simulate this
by placing ice on the bimetallic strip when the light (air conditioner) is on.
Safety Considerations.
The bi-metallic metal may have sharp corners and jagged edges. Be careful of
cuts. Using matches will increase the risk of fire, be careful.
1. Why must bi-metallic strips be made of two different types of metal?
2. When heated and the bi-metallic strip bends, which metal is on the outside
of the bend? The one that has the greater expansion, or the one with the
lesser expansion?
3. Why is it best to have the second nail as far from the cork as possible?
4. When using the bi-metallic thermostat to control heating, should the side of
the bi-metallic strip with the greater expansion be touching the second nail,
or should it be away from the second nail?
5. Name some uses for bi-metallic metals in your home (remember, almost
anything that has a temperature setting, has a bi-metallic metal part).
Bi-metallic metal can be purchased from Crest Manufacturing Company