Radiometer
Radiometer (rā`di*om"e*tẽr) , noun
[Latin radius radius + -meter: compare French radiomètre.]
1.
(Nautical) A forestaff.
2.
(Physics) An instrument designed for measuring the mechanical effect of radiant energy.
It consists of a number of light disks, blackened on one side, placed at the ends of extended arms, supported on a pivot in an exhausted glass vessel. When exposed to rays of light or heat, the arms rotate.