In 1783, Horace Bénédict de Saussure built the first hygrometer, a device to measure humidity, and he built it with… hair. To understand how it worked, you need to know a thing or two about hair.

A single strand of hair has many layers. The inner layer is filled with proteins called keratins that bind to each other, giving shape to your luscious locks. These proteins bind by forming tough disulfide bonds or weaker hydrogen bonds. You can thank hydrogen bonds for the funny way your hair dries naturally after getting out of the shower. Water molecules (two hydrogens and an oxygen) are soaked up by your hair and act as a bridge linking keratin molecules together in place. These hydrogen bonds keep your hair fixed in shape until you wet it again, allowing new hydrogen bonds to form.

In high humidity, water molecules in the air find their way into straight strands. As hydrogen bonds connect keratin proteins, hair starts to fold back on itself and curl. Frizzy fly always occur when hair folds back enough to break the cuticle – or the outer layer of hair that looks like dragon scales under a microscope.

Enter hygrometer. Saussure attached one end of a 10-inch piece of human hair to a screw. The rest of the strand he maneuvered through a pulley and attached to a weight. As the hair took on moisture, the strand curled and shortened moving the pulley and lifting the weight. Saussure could then calculate how much humidity was in the air based on how much the weight moved.

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