If you can remember thermostats before they went ‘digital’, then you may know about bi-metallic strips – a piece of material made from of two strips of different metals which expand at different rates as they are heated (usually steel and copper). The result is that the flat strip bends one way if heated, and in the opposite direction if cooled.
Normally, it takes serious skill and a red-hot torch to stick two different metals together, but here’s a homemade version of this concept that your kids can make using your freezer. Here what you do:
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Materials:
- foil wrapper from a stick of gum or candy bar
- index card
- scissors
- tape
Since gum wrappers are paper on one side and foil on the other, you can use one to make your own bi-metallic strip. Flatten out the wrapper into a sheet and find a way fasten the wrapper so it sits upright on an index card (we used the bubble gum itself as the adhesive). Stick it in the freezer overnight and check it in the morning! Where can you place it to flex the other direction?
How does that work? A bimetallic strip is a stack of two metals stuck together. The metal with the higher expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled. The bi-metallic strip was invented by the eighteenth century clockmaker John Harrison to compensate for temperature-induced changes his clock springs.
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It will bend when the air changes in temperature. It is actually best if the strip does not touch anything (except where it is taped to the card).
Does it bend when the paper/foil touches something cold/warm? or when the air is cold/warm?