This is a working hygrometer by F. Darton & Co. Ltd of Watford circa 1970. Their trade catalogue describes the instrument as 'a pocket (paper) hygrometer, range 20 - 100% R.H., comprising paper and metal coil elementin 3" diameter plastic case with bevelled glass front. Black anodised dial with legible scales in red and white, figured every 10% subdivided every 5%, estimable to 2%'. In good condition.
Hygrometer by Darton of Watford