A hygrometer is a device designed specifically to do just one thing and that’s measure the amount of moisture in the air. While there are many uses for hygrometers, guitar players often rely upon them to determine whether their instrument(s) are being stored in an environment that is neither too moist nor too dry. The first hygrometer was actually built by John Frederic Daniell way back in 1827, when he first published a paper on measuring humidity. He is best known for the electro-chemical cell, which he devised in 1836 and which was the inspiration for all such devices to come. Today there are compact hygrometers which can be set in a room where guitars are being stored to measure humidity, and even smaller units which can be placed in a guitar case to monitor moisture levels for each instrument. The Parker PJ24 ships in a case with a built-in hygrometer.