Psychometer

Psychometer is not used to check the mental state of patients, but to measure the humidity of the air. It is a simple device made from two thermometers, some gauze, and a water container.

The principle of operation is very simple. One of the thermometers measures the temperature of the environment. The other is covered with gauze which is saturated with water from the container. The water vapours from the gauze taking the heat from the container. As a result, the temperature on the thermometer falls. This process is counterbalanced by the heat provided from the environment. This results in the thermometer showing a certain temperature after the amount of heat consumed in vaporization has been balanced with the amount of heat received from the environment. The rate of vaporization depends on the amount of water vapour present in the air. That is why the temperature drop depends on relative humidity.

Thus, in dry air, when you put a wet towel on your head, it will help you cool your hot head. While in a sauna even a wet towel will not help.