Hygrometers are instruments used for measuring humidity. The simplest form of a hygrometer consists of two thermometers, one of which has its bulb constantly kept wet. Evaporation from the bulb lowers the temperature so that this thermometer shows a lower temperature. Humidity is computed from the ambient temperature as shown by the "dry bulb thermometer" and the difference in temperature shown by the "wet bulb" and dry bulb thermometers. One device that uses the wet/dry bulb method is the sling psychrometer, where the thermometers are attached to a handle or length of rope and spun around in the air for a few minutes.
There are several other types of hygrometers that are commonly used. Frequently, the devices use the stretching of a human or animal hair under tension to determine the ambient humidity. In order to see changes that occur over time, many hygrometers record the value of humidity on a piece of graduated paper so that the values can be read off the chart. However, modern instruments often use electronic means of recording the information.
- USATODAY.com: How a Sling Psychrometer Works