From AMS Glossary
(Also called actinic balance.) An instrument that measures the intensity of radiant energy by employing a thermally sensitive electrical resistor; a type of actinometer.
In meteorological applications, two identical, blackened, thermally sensitive electrical resistors are used in a Wheatstone bridge circuit. Radiation is allowed to fall on one of the elements, causing a change in its resistance. The change is a measure of the intensity of the radiation.