I1-14: THERMAL EXPANSION OF ALUMINUM - OPTICAL LEV

ID Code: I1-14
Purpose: Demonstrate thermal expansion in a complicated way.
Description: A small mirror with an iron lever attached to it rests on one end of an aluminum tube. The other edge of the lever rests on the pole of a magnet. When the aluminum tube is heated with a burner the mirror is deflected. To measure this deflection a laser beam is reflected off the mirror onto a scale as shown in the picture. When the tube is heated the laser spot is deflected about 50 cm when the distance between the mirror and the scale is about 2 m. The picture at the right is a detail of the mirror mount.
Availability: Available

Interestingly, the material cools and shrinks much more slowly than it heats and expands, It typically heats up enough to deflect the beam noticeably in only a few seconds, but may take 15-10 minutes to return to its starting position.
What is the primary cause of this difference? Would it be interesting to have students discuss why this is?