Final answer:
A thermometer uses the expansion of a substance, such as mercury, to measure temperature, while a hygrometer measures humidity using a hair that expands or contracts with changes in moisture.
Step-by-step explanation:
The working principle of a thermometer is based on the fact that substances expand or contract with changes in temperature. A common type of thermometer, called a mercury-in-glass thermometer, uses the expansion of mercury to measure temperature. As the temperature rises, the mercury in the bulb expands and rises up a graduated scale.
A hygrometer, on the other hand, measures humidity, which is the amount of moisture present in the air. There are different types of hygrometers, but one common method is using a hair hygrometer. This device uses a human or animal hair that expands or contracts with changes in humidity. The hair is connected to a pointer that moves on a scale, indicating the humidity level.