From AMS Glossary
An absolute pyrheliometer, developed by C. G. Abbot, in which the radiation-sensing element is a blackened water-calorimeter.
It consists of a cylinder, blackened on the interior, and surrounded by a special chamber through which water flows at a constant rate. The temperatures of the incoming and outgoing water, which are monitored continuously by thermometers, are used to compute the intensity of the radiation. This instrument was built by the Smithsonian Institution but was never widely used as a standard instrument. The function of the instrument as an absolute reference is currently fulfilled by the absolute cavity radiometer.