This is an instrument equipped with two thermometers, side by side.
The bulb of one of the thermometers is covered with a muslin wick dipping into water.
The water is absorbed up the muslin and evaporates, cooling.
The difference between the wet and dry bulb temperatures is called the wet bulb depression.
If the air is fully saturated (100% relative humidity) the water cannot evaporate, so both the wet and dry bulb temperatures are the same.
Hair Hygrometer
These devices use a human or animal hair under tension.
The hair is hygroscopic (tending toward retaining moisture); its length changes with humidity, and the length change may be magnified by a mechanism and indicated on a dial or scale.
The traditional folk art device known as a weather house works on this principle. Whale bone and other materials may be used in place of hair.
This instrument uses strands of human or horse hair with the oils removed attached to levers that magnify a small change in hair length.
