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Q: Unusual toys ( Answered 5 out of 5 stars,   0 Comments )
Subject: Unusual toys
Category: Sports and Recreation > Toys
Asked by: jimheard-ga
List Price: $5.00
Posted: 17 Dec 2002 13:39 PST
Expires: 16 Jan 2003 13:39 PST
Question ID: 126147
Where can I find the following toy: it's a glass bulb, about the size
and shape of a standard light bulb, that contains a set of paddles,
mounted on a thin metal post (like a needle).  There are four paddles,
two each at opposite ends of thin metal, perpendicular arms.  The
paddles are black on one side, white on the other.  They spin when
exposed to light.
Subject: Re: Unusual toys
Answered By: leli-ga on 17 Dec 2002 14:21 PST
Rated:5 out of 5 stars

This toy is called a radiometer or 'light windmill'.

First I found three places where you can buy the standard version,
usually sold as a scientific toy for children. (Prices not including

Sundance Solar  $9.95

Museum of Science and Industry store  $10.95

Star Gazers Gift Shop: $12.95  (scroll down the page)

Then I found Sunspot Solar who have a surprising variety of sizes and

They also offer interesting information on these 'light windmills':

"The radiometer, also known as a light windmill, is an instructive
physical object for demonstrating a theory that was invented by the
English physicist Sir William Crookes in 1873. This small physical and
technical miracle shows, in a simple way, how the “smallest sun power
plant in the world” converts light into energy. It also displays how
we might be able to use the sun as a source of energy for the future."

And here's their explanation of how it works:

"When warm light, that is sunlight or the light of electric bulbs (but
not cold neon light) strikes the wings resting on the needle in the
radiometer bowl they start to turn.  Depending on the power of the
light, the wings turn at varying speeds. Inside the glass bowl there
is a partial vacuum produced by the air being thinned in order to keep
the balance between air resistance and the turning power of the solar
energy. The black surfaces of the wings absorb more light than the
white surfaces, therefore the pressure on the black surfaces is much
stronger than on the white surfaces. The more the molecules in the
bowl heat up, the stronger the pressure on the black surfaces and the
wings start to turn steadily. Depending on the power of the light, a
rotation of 3,000 revolutions per minute is possible."

If you are interested in the pricier, decorative radiometers you'll
find more places offering them via this search:

I hope you (or the person you're giving this to) will enjoy the
radiometer. Please feel free to ask for clarification if I can assist
further with this.

Regards - Leli

Clarification of Answer by leli-ga on 18 Dec 2002 00:32 PST
Thank-you very much for the generous comments and rating. 
I'm glad to have been able to help.
jimheard-ga rated this answer:5 out of 5 stars
I am truly amazed at the speed and quality of your research.  I am an
attorney (Stanford Law School), and did my own research on the web
(and by calling and visiting stores) before turning to you in
desperation.  Thanks so much for a fantastic job!

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