Psychrometer
Use
Indirect measurement of the relative humidity of the atmosphere.
Description
It is made of two mercury thermometers, one of them is wrapped by a piece of cotton that is inserted, on the other end, into a container filled with water.
Functioning
The dry bulb thermometer shows the air temperature at every moment and it always sets a value that is higher than that of the wet bulb thermometer.
The cause of this phenomenon is that the evaporation of water ( in the cotton) is an endothermic process: it absorbs a certain amount of heat from mercury, so the temperature of the mercury comes down. The greater the amount of water evaporated, the greater the amount of heat absorbed and, consequently, the decline in temperature.
On the other hand, as the intensity of the evaporation depends on the amount of water in the air, we can calculate the relative humidity from the temperature difference between both thermometers (using tables or graphs incorporated in the device).
History
The first version of the psychrometer, dated in 1810, was made by the Scot Sir John Leslie, although it was upgraded by the French Gay-Lussac (1822) and August (from 1825 to 1848).
