The moisture content of air is a significant variable not only in air conditioning for human comfort but also in industrial processing and manufacturing plants, where it can influence the quality of the product. In the spinning of textiles, for example, better results are obtained when humidity is controlled at optimal values. In drying operations, also, the moisture-laden air from the drier reflects the amount of moisture removal from the solid, and its moisture content can be used as the basis for operation of the drier. Similar needs for moisture measurements are found in other fields, such as in metal heat treating under protective atmosphere, where the moisture content can be quite critical, and in the manufacture of various industrial gases, where absence of moisture in the gas is important. The three principal methods of measuring relative humidity (RH) use nylon hygrometers, wetand dry-bulb thermometers, and dew-point measurement.