Radiometer

A classic top-selling item!
Perhaps the best and most reliable way to demonstrate radiant heat! 
Partially evacuated glass bulb contains spinner with four vanes, blackened on one side, silvered on the other. Turns 3,000 revolutions a minute in sunlight. Will also turn under an incandescent lamp (100 watts or more). The stronger the wattage, the faster it will turn. 
Detects and demonstrates energy of heat radiation.