A Thermo-Hygrograph or hygrothermograph is a chart recorder that measures and records both temperature and humidity (or dew point). Similar devices that record only one parameter are a thermograph for temperature and hygrograph for humidity. Uses a bi-metallic strip for temperature and one human hair bundle for humidity. Includes one red and blue felt–tipped pen to record the reading.
A thermograph is usually configured with a pen that records temperature on a revolving cylinder. The pen is at the end of a lever that is controlled by a bi-metal strip of temperature-sensitive metal which bends as the temperature changes.