(1)Each hygrometer shall comprise two mercurial thermometers of wet-bulb and dry-bulb of similar construction, and equal in dimensions, scale and divisions of scale. They shall be mounted on a frame with a suitable reservoir containing water.(2)The wet-bulb shall be closely covered with a single layer of muslin kept wet by means of a wick attached to it and dropping into the water in the reservoir. The muslin covering and the wick shall be suitable for the purpose, clean and free from size or grease.(3)No part of the wet-bulb shall be within 3 inches from the dry-bulb or less than 1 inch from the surface of the water in the reservoir and the water reservoir shall be below it, on the side of it away from the dry-bulb.(4)The bulb shall be spherical and of suitable dimensions and shall be freely exposed on all sides to the air of the room.(5)The bores of the stems shall be such that the position of the top of the mercury column shall be readily distinguishable at a distance of 2 feet.(6)Each thermometer shall be graduated so to that accurate readings may be taken between 50 and 120 degrees.(7)Every degree from 50 up to 120 degrees shall be clearly marked by horizontal lines on the stem, each fifth and tenth degree shall be marked by longer marks than the intermediate degree and the temperature marked opposite each tenth degree, i. e., 50, 60, 70, 80, 90, 100, 110 and 120.(8)The markings as above shall be accurate, that is to say, at no temperature between 50 and 120 degrees, shall the indicated readings be in error by more than two-tenths of a degree.(9)A distinctive number shall be indelibly marked upon the thermometer.(10)The accuracy of each thermometer shall be certified by the National Physical Laboratory, London, or some competent authority appointed by the Chief Inspector and such certificate shall be attached to the Humidity Register.
