Pat Shingleton: "Rittenhouse and His Invention..."
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 archaic device to register the expansion and contraction of wood detected through the wood’s grain. Wood swells and shrinks about 80 times as much around the growth rings and 40 times as much across the rings. His process included taking two identically sized strips of mahogany, 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 swelled, forcing the slat to bend and when it dropped the strip shrank and bent. 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. As wood swells and shrinks it gains or loses water once it is at or below a 30% saturation point. Examples include gaps between floorboards, creaking stairs, trim joints opening up, piano sound boards changing tunes with doors and drawers that once stuck, opening easily. Of course the process reverses when higher levels of relative humidity restores moisture content to its original levels. Rittenhouse also made mathematical instruments for surveying and astronomy and in the 1700s invented the Rittenhouse Hygrometer. This weather instrument registers the relative humidity of the air by using wood as its sensor. The weatherstick was invented in the late 1700s and is still used to forecast the weather.