radiometer
    n 1: meter to detect and measure radiant energy (electromagnetic
         or acoustic)

Radiometer \Ra`di*om"e*ter\ (r[=a]`d[i^]*[o^]m"[-e]*t[~e]r), n.
   [L. radius radius + -meter: cf. F. radiom[`e]tre.]
   1. (Naut.) A forestaff.
      [1913 Webster]

   2. (Physics) An instrument designed for measuring the
      mechanical effect of radiant energy.
      [1913 Webster]

   Note: 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.
         [1913 Webster]