What is the purpose of the sling on a psychrometer?

The purpose of the sling on a psychrometer is to facilitate the process of measuring humidity levels in the air by swinging two thermometers—one dry bulb and one wet bulb—through the air. This action promotes evaporation of water from the wet bulb's surface, which causes a temperature drop that can be read on the thermometer. The difference between the readings of the two thermometers is then used to calculate the relative humidity. This method relies on the principle that evaporation has a cooling effect, which is vital for determining moisture content in the air effectively.

Other methods listed, such as calibrating the device, measuring pressure, or measuring temperature directly, are not the primary functions of the sling on a psychrometer, underscoring the specific role of the swinging action in obtaining accurate humidity measurements.