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The Radiometer

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-256

2025-12-30

899

 is an instrument used to detect radiant heat. It consists of a glass bulb inclosing two arms of aluminum crossing each other at right angles and carrying at each end a mica vane, one side of which is coated with lampblack while the other is bright. These arms are fastened horizontally to a vertical shaft which can rotate with them.

The air is nearly exhausted from the bulb. When these vanes are subjected to the action of radiant heat, they set up a rotation, the velocity of which is dependent upon the intensity of the heat received.

This is due to the fact that the molecules of rarefied air in their vibration bound back and forth between the vanes and the FIG. 1 walls of the bulb; that the blackened side of each vane becomes warmer than the other; and that the molecules rebound from the warmer side with greater speed, producing a greater reaction of pressure on that side.

Demonstration. - Make a lighttight box, large enough to hold a radiometer. Heat a flat piece of cast iron or brass nearly to a red heat and fasten it to the inner face of the door of the box (Fig. 1). Put the radiometer into the box and close the door. Leave it closed for about a minute and on opening it the radiometer will be found to be rotating. Is it heat or light that produces the rotation?