Weather instruments have not changed too much over the years. The instrument on the left is a mercurial barometer. On the right is a sling psychrometer.

From The Aims and Methods of Meteorological Work by Cleveland Abbe. In: Maryland Weather Service, Johns Hopkins Press, Baltimore, 1899. Volume I.

A psychrometer is an instrument commonly used in laboratories to measure relative humidity. It is also referred to as a wet- and dry-bulb thermometer.

This instrument consists of two similar thermometers that are mounted side by side. The dry bulb has its bulb exposed to the air. The wet bulb is wrapped in an absorbent material such as muslin, which is immersed in water and serves as a wick. When the web bulb is taken out of the water, it cools by evaporation of the water. If the bulb is whirled around to hasten evaporation, it is called a sling psychrometer. If air is forced past the bulb, it is referred to as an aspirated or ventilated psychrometer.

The amount of evaporation, and consequent cooling of the thermometer, depends on the humidity of the atmosphere—the drier the atmosphere, the faster the water evaporates. Using this data and humidity tables or calculations, the dew point (the temperature to which air would have to be cooled for saturation to occur) can be determined, and from it, the relative humidity.