Henry Novakovich, age 13, of Springfield, I11., for his question:

WHAT IS A RADIOMETER?

A radiometer is a device for measuring the intensity of radiant energy. It converts radiation of mostly unknown composition into heat and measures it.

A common form of radiometer consists of a glass bulb which encloses a rotation shaft carrying vanes. Almost all air is removed from the bulb. One side of each vane is painted black and the other silver.

When radiant energy falls on the vanes, it heats their blackened sides more than their silvered sides. Air molecules meeting the black sides take up more energy than those meeting the silvered faces. The molecules on the vane's blackened sides recoil harder than those on the silvered sides. This causes the blackened sides of the vanes to move away from the radiation.

If all the air were removed from the bulb and the friction at the ends of the shaft were reduced, the silvered sides of the vanes would move away from the radiation. This happens because radiation has a slight pressure.

The speed at which the vanes of the radiometer spin shows the intensity of the incoming radiant energy.