The sling psychrometer has twothermometers. The purpose of both is to get both the air temperature, and the'wet-bulb' temperature, which can be used to deduce the dewpoint temp andtherefore relative humidity.
One thermo. simply measures the airtemperature and the other is more complicated. The bulb of the other is wrappedin some gauze, and the gauze is imbued with a little water, or sometimes theyuse rubbing alcohol. The rubbing alcohol is used instead if it's below freezingand the water would freeze.
You swing the contraption (some have alittle fan instead to blow across the bulbs). The air flowing over the one withgauze/alcohol will have evaporation going on around the bulb. The temperaturethat results on the bulb with the evaporation is called the wet-bulbtemperature. This is NOT the dewpoint. The wet-bulb temp is in between thedewpoint (the point at which water vapor in the atmosphere can startcondensing) and the regular air temperature. The reason it's not the dewpointis because as water evaporates, it gives off heat. So the result is that itshows a temp higher than the actual dewpoint.
It's true though that the higher thewet-bulb temp, the more humidity/moisture is in the air.
Meteorologists and weather observers use a table tolook up the corresponding dew point using the dry air temp (dry bulb temp) andthe wet-bulb temp . There are math formulas, often simplified into more lookuptables, for dry bulb temp, DEWPOINT, and the corresponding relative humidity.And that's what they do to determine the dewpoint and relative humidity using asling psychrometer.