MACHINES THAT SEE
Much more has been done with machines that see. There is a delicate device known as a “sun valve,” which is used on beacon lights so that as soon as it grows dark or very foggy the lamp is automatically lighted, and when the day dawns or light breaks through the fog the light is extinguished. The sun valve has two rods, one brightly polished and the other a dead black. Light and its attendant radiant heat waves are absorbed by the black rod, but are reflected from the bright rod. As a consequence, the black rod grows hotter than the polished rod and expands. The difference of expansion between the two rods is utilized to operate a valve which controls the supply of gas to the lamp. This valve is very sensitive and marvelously responsive to variations of light.
There is a chemical element called selenium, which is peculiarly sensitive to light. When light shines upon selenium its electrical resistance is lowered, and hence it can be used as a light-operated valve to control the flow of electric current.