A mechanical hygrometer is a device used to measure the humidity level in the air. It typically operates using materials that change shape or size in response to moisture. Commonly, these hygrometers use a strand of human or animal hair, which expands or contracts based on humidity changes, providing a reading on a dial. These instruments are often found in homes, greenhouses, and laboratories, helping to monitor conditions for comfort or specific processes. While they are less common than digital hygrometers, mechanical hygrometers are valued for their simplicity and reliability in measuring relative humidity without the need for batteries.