Who invented the first crude hygrometer?

A hygrometer is an instrument used to measure the moisture content – that is, the humidity – of air or any other gas. A first crude hygrometer was invented by Leonardo da Vinci in 1480.

On the quadrant hinges a bar, at the extremities of which a sponge and a block of wax are attached: one is impermeable (the wax), while the other is hygroscopic (absorbent: the sponge or cotton) and weighs more in accordance as the air is more humid, causing the balance pans to incline and the bar to move along the graduated quadrant.

Francesco Folli invented a more practical version of the device. Robert Hooke is noted as the first mechanical hygrometer as it measured the way in which an oat husk would change with the relative humidity. In 1783, Swiss physicist and Geologist Horace Bénédict de Saussure invented the first hygrometer using human hair to measure humidity.

Today, many of the humidity devices that we calibrate fall into the category of psychrometers, which measure humidity using a pair of mercury thermometers. While one thermometer is dry, the other has a wetted base and is ventilated. The water causes the reading to drop as it interacts with heat, and the differing readings can be interpreted to discern the relative humidity of the environment.

Leonardo di ser Piero da Vinci (15 April 1452 – 2 May 1519) was an Italian polymath of the High Renaissance who was active as a painter, draughtsman, engineer, scientist, theorist, sculptor, and architect.