An absolute instrument developed by K. Ångström for the measurement of direct solar radiation. The radiation receiver station consists of two identical manganin strips, the temperatures of which are measured by attached thermocouples. One of the strips is shaded, while the other is exposed to sunlight. An electrical heating current is passed through the shaded strip to raise its temperature to that of the exposed strip. The electric power required to accomplish this is a measure of the solar radiation. See actinometer, pyrheliometer; compare Ångström pyrgeometer.