The instrument consists of two blackened platinized manganin strips, the temperatures of which are being measured. When operating, one of the strips is shaded, while the other is exposed to sunlight. An electric current is passed through the shaded strip to raise its temperature to that of the exposed strip, and the required electric power is a measure of the solar radiation. As the instrument has a higher accuracy, it was used as a standard for calibration of other solar radiation instruments.