The Probert Encyclopaedia of Science & Technology

RADIOMETER

A radiometer is an instrument for measuring the mechanical effect of radiant energy. It consists of four crossed arms of very fine glass supported in the centre by a needle-point, and having at the ends thin pith discs blackened on one side. The instrument is placed in a glass vessel exhausted of air and when exposed to light the arms revolve. It was invented by Crookes in 1874 and forms a popular amusement today.
Research Radiometer