Wide range of solar radiometers, vanes in glass globe turn in light. Put them in a sunny window, watch the vanes turn. Great gift idea!
Sunlight, or heat from a flame, or even your hand, can make the vanes rotate. If the radiation is strong enough, according to one source, the vanes can rotate at up to 3,000 times/minute. How do they know this? Well, I have found a method, using a laser pointer, a light-dependent resistor (LDR), a battery, and two loudspeakers and oscillator. Shine the laser through the radiometer, onto the LDR, such that the vanes interrupt it as they rotate. The LDR connects to the speaker and battery in series. Bring a bright lamp close to the radiometer; as the vanes rotate faster you can hear a low hum from the speaker, rising in pitch. Compare this with the oscillator connected to its own speaker; you can get the same frequency from each, and read it off from the oscillator. Divide by 4 because the radiometer has 4 vanes, and you have the rotation speed.
