A hygrometer using goldbeater's skin as the sensitive element. Variations of the physical dimensions of the skin caused by its hygroscopic character indicate atmospheric relative humidity. The length of a piece of goldbeater's skin changes between 5% and 7% for a change in humidity from 0% to 100%. The time constant of response becomes extremely long both at low ambient temperatures and at very high and very low relative humidity. (Note: Goldbeater's skin is the prepared outside membrane of the large intestine of an ox; it is used in goldbeating to separate the leaves of the metal. )