Wood swells and shrinks about 80 times as much around the growth rings relative to the same section of wood along the length of the fiber. This causes wood of certain cuts to warp when exposed to high humidity. As a clockmaker, this phenomenon greatly frustrated David Rittenhouse so he sought to understand it.
Taking two identically sized strips of mahogany and gluing them together so that the grains of one side of the slab are perpendicular to the grain of the other, Rittenhouse built a device that moved as the ambient humidity in the air changed. When the humidity rose, the strip swelled, forcing the slat to bend and when it dropped the strip shrank and bent in the other direction. Rittenhouse’s invention is still used today by amateur weather watchers although the material that expands and shrinks is now wool or horse hair instead of wood.
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Michael Carver -- Historical Interpreter/Reenactor 