Demos: 3A-03 Bimetallic Strip

Two dissimilar metals, bonded together securely, are heated in the flame of a burner. Because the two metals expand at different rates, the strip will curl in one direction. Upon cooling, it will return to its initial state. The principle is then applied to a thermostat made of bimetal. Upon heating by a light bulb, the thermostat causes a bell to ring.

Directions: Hold the strip flat-side down in the flame. (Beforehand, determine which orientation will cause the strip to bend downward.) Allow the strip to cool. Then turn the blade over and reinsert it in the flame. The strip will now bend upward.

For the thermostat demo, simply turn on the switch. The light bulb will warm the bimetallic strip, causing it to bend toward a contact. When contact is made, the bell rings.

Suggestions for Presentation: Make sure you have the strip oriented so that it will bend downward. Then place it in the flame and tell the students that the flame is very hot and will soften the metal, causing it to sag! Then say, “Let’s see that again.” Subtlely turn the strip over and reintroduce it into the flame. Now it will bend upward. Ask the students to explain what is going on.

Applications: Bimetallic strips are the heart of many thermostats.

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