Leonardo da Vinci designed a hygrometer to measure humidity using a balance.
His concept used a material that changes weight when it absorbs moisture from the air. The material was balanced against a known weight, so changes in the balance could indicate differences in atmospheric humidity. This made the device an early attempt to quantify a property that people could otherwise only sense.
Leonardo’s hygrometer belongs to his broader investigations of weather, air, water, and measurement. It was a scientific instrument concept rather than a modern electronic sensor, and its readings would not have had the precision or standardized calibration expected today.
The instrument is often confused with a barometer. A hygrometer measures moisture in the air, while a barometer measures atmospheric pressure. A thermometer measures temperature, and an anemometer measures wind speed. Those distinctions make “hygrometer” the unique answer to this question.