Background
To understand how a hygrometer made with hair strands works, it is important to understand the structure of a shaft of human hair. (The shaft is the part of the hair that sticks out above the surface of the skin, whereas the hair follicle is the part that is below the skin's surface.) In human hair shafts, several outer layers of flattened cuticle cells surround a layer of cortical cells (making up the cortex), which surrounds the central core of cells, called the medulla. The flattened cuticle cells are relatively tough and have a scalelike appearance when magnified, along with a coating of specialized molecules (called lipids) that repels water, helping to protect the hair strand. The cortical cells are fibrous and contain pigment granules of varying darkness that give hair strands their natural color.
The predominant proteins in hair are from a group called keratins, from the same protein family that forms your fingernails. The chemical bonds link the keratin molecules are what give a hair strand its strength and flexibility as well as its predictable response to humidity.