Expanding Metal

Take an empty, corked wine bottle, push as long an aluminum knitting needle as you can find into the bottle cork and let the other end project under slight pressure over the mouth of a second, uncorked bottle. Glue a paper arrow on to a sewing needle, making sure that it is balanced, and fix it between the knitting needle and the neck of the bottle. Place a candle so that the tip of the flame touches the middle of the needle and watch the arrow.

The arrow turns quite quickly some way to the right because the knitting needle expands on heating like other substances. With an ordinary steel knitting needle the arrow would only turn a little, because steel only expands half as much as aluminum. Since the aluminum is longer as well, the difference is still greater. The expansion is clearly visible in electricity power cables, which sag more in summer than in winter. If you take the candle away from the knitting needle, the arrow moves back.