Lithium chloride hygrometer
A formerly used type of hygrometer that measured humidity based on the function of temperature and electrical conductivity of a hygroscopic salt. Most lithium chloride hygrometers contain a bundle of glass fibers coated with a dry film of lithium chloride salt. The fibers are wrapped with metal wires connected to a continual ac source. When the fibers are dry they are nonconductive. When the fiber surfaces absorb moisture, they become conductive allowing a current to pass through the wire. This current heats the fibers which in turn drives off the moisture from the salt. Thus the wires are kept at the temperature required to just maintain dryness. This temperature can be directly converted to humidity. Lithium chloride hygrometers can take 3-10 minutes to respond to environmental changes (Thomson 1978). They require a continual electrical source.
G.Thomson, The Museum Environment, Butterworths, London, 1978.