An instrument developed by K. Ångström for measuring the effective terrestrial radiation. It consists of four manganin strips, of which two are blackened and two are polished. The blackened strips are allowed to radiate to the atmosphere while the polished strips are shielded. The electrical power required to equalize the temperature of the four strips is taken as a measure of the upwelling terrestrial radiation. See actinometer, pyrheliometer; compare Ångström compensation pyrheliometer.