Hygrometer

This is a device specifically designed to measure the relative humidity of the air in any given environment. Often, these are built in tandem with thermometers. For musicians, hygrometers are important, mainly for the long-term protection of wood instruments, since humidity that is too high or too low can eventually damage any wood instrument, including guitars, mandolins, violins, or cellos. Most modern hygrometers use electronic sensors to measure humidity, with the two most common sensors being capacitive and resistive. A few manufacturers actually build hygrometers into their guitar cases.