Final answer:
A two-bulb hygrometer measures humidity by comparing temperatures between a dry bulb and a wet bulb, where the evaporation rate on the wet bulb, which affects its cooling, indicates the air's humidity level.
Step-by-step explanation:
The working principle of a two-bulb hygrometer is based on the measurement of the difference in temperature between two thermometers, one with a dry bulb and one with a wet bulb. The dry bulb thermometer measures the ambient temperature, while the wet bulb has a cloth soaked in water wrapped around its bulb, causing evaporation which cools the thermometer. The evaporation rate, and thus the cooling effect, is dependent on the humidity of the air; drier air leads to more evaporation and greater cooling. This temperature difference is used to calculate the relative humidity of the air.