How many thermometers does a sling psychrometer use to measure relative humidity?

A sling psychrometer utilizes two thermometers to measure relative humidity. One thermometer, known as the dry bulb, measures the actual air temperature, while the other, referred to as the wet bulb, has a wick that is moistened and is used to measure the cooling effect of evaporation.

When air moves over the wet bulb, moisture evaporates, cooling the thermometer. By comparing the readings from both thermometers, it is possible to determine the relative humidity of the air. This dual-thermometer system is essential, as the difference in temperature between the dry and wet bulbs is used to calculate relative humidity using standard psychrometric charts or formulas.

The option that suggests using only one thermometer would not be adequate for measuring humidity, as it would not account for the effects of evaporation that are crucial for determining relative humidity. Similarly, options indicating three or four thermometers are unnecessary for this measurement, as the wet and dry bulb readings are sufficient to derive the required information.