Also called a wet-and-dry-bulb thermometer, a psychrometer is
a simple type of hygrometer, an instrument that measures temperature
and humidity. The psychrometer consists of two thermometers,
one with a dry base and one with a wet base, mounted beside
each other in a frame. The bulb of the wet thermometer is wrapped
in cloth that's been soaked in water. As the water evaporates,
it absorbs heat, which causes the thermometer's temperature
reading to drop. The drier the air, the quicker the water will
evaporate and the greater the drop in the thermometer's temperature
reading. The difference in readings between the wet and dry
thermometers can be used to determine the relative air humidity.
This model, made by Taylor, is called a sling psychrometer.
It is held by the handle and spun around to hasten evaporation
from the cloth, as demonstrated below.