The hair hygrometer is used to indicate the relative humidity by means of changes in length a hair undergoes with changes in atmospheric humidity. It's easier to use than the psychrometer, though it is less accurate.
It consists of a long hair degreased with ether, kept in tension by a pair of tweezers at the top of the instrument. Tweezers can be raised or lowered by means of a screw, placed above the tweezers.
In this way the hair is not knotted to the suspension, otherwise a twist would be introduced making irregular the elongation.
The lower end of the hair is wound on one of the two grooves of a pulley at the base of the instrument and in the second groove is wrapped a silk wire. The latter is connected to a counterweight that keeps the hair under tension.
The metal plate and the pulley are bolted to a sturdy metal frame, which can be suspended with its top ring, or can be attached to a heavy wooden or metal base.
To the axle of the pulley is fastened a light needle, which ends along a graduated scale. With increasing moisture the hair elongates and the needle drops. When humidity decreases the hair shortens, the pulley turns in the opposite direction and the needle rises.
The instrument is equipped with a thermometer to measure the temperature.