The Rittenhouse Hygrometer was an 18th century invention that registered the relative humidity of the air by using wood as its sensor. David Rittenhouse created this device to register the expansion and contraction of wood detected through the wood’s grain.  If you've ever experienced tree removal a cut tree will display tree rings that are a somewhat accurate measure of a particular tree's life span. Each ring registers to approximately a year. Thicker rings often indicate wetter yearly conditions, thinner rings, dry spells. Wood swells and shrinks about 80 times as much around the growth rings and 40 times as much across the rings.  Rittenhouse took two identically sized strips of mahogany and glued them together to complete a single slat and attached one end to a base. He then placed a tipped pointer on the other end.  When the humidity rose, the strip would significantly swell and forced the slat to bend.  A decrease in humidity found the strip dropping as it would shrink and and bend.  Rittenhouse’s invention is still used today by designers of plywood, laminated floors and layered wood to ensure that these products remain flat as they adjust to the power of relative humidity.