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What is a Crookes radiometer?
A Crookes radiometer is a glass bulb, partly evacuated, holding four vanes on a pin, each black on one side and silver on the other. Put it in sunlight or under a warm lamp and the vanes spin, faster in stronger light; in the dark they stop. William Crookes built it in 1873, and it is still the simplest visible proof that light carries energy. No batteries, no switch, no maintenance.
5 modelsFrom £43.99Hand blown glassNo batteries, runs on lightFree UK delivery over £45
Who is this collection for?
☀️
For a sunny windowsill
A Crookes radiometer spins in light and stops in the dark. No batteries, no switch, just the sun. The oldest solar toy there is.
🎁
For a scientist, a teacher or a curious mind
Small, elegant, Victorian. The gift that comes with a physics puzzle: why does it spin the way it does?
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For lovers of scientific instruments
Hand blown glass and a spinning vane in a partial vacuum. A desk object with 150 years of history.
How to choose a radiometer
There is little to choose except size and stand: a classic 15 cm glass radiometer for a windowsill, a larger one for a shelf, and models with a wooden or metal base for stability.
| Type | Best for | Height | Price |
|---|---|---|---|
| Classic Crookes radiometer | Windowsills, desks, first one | 15 to 18 cm | £44 to £50 |
| Solar radiometer with base | Shelves, gifts, stability | 18 to 22 cm | £50 to £60 |
| Large glass radiometer | Classrooms, display | 22 cm and over | £60 to £68 |
It is thin glass on a stem: treat it like a light bulb. If it stops spinning, the vane has usually slipped off its pin; a gentle tilt reseats it.
The science in two lines
It is not light pressure, which would push the silver side. The black sides absorb light and warm up; the thin gas left in the bulb slides along the vane edges from cold side to warm side (thermal transpiration) and pushes the vanes with the black faces trailing. Reynolds and Maxwell worked this out in the 1870s.
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Radiometer: your questions, answered
How does a radiometer work?
Light warms the black sides of the vanes more than the silver ones. The thin gas left in the bulb pushes harder on the warm side, and the vanes turn.
Does it work with a lamp?
Yes, with a bright incandescent or halogen bulb close by. LED and fluorescent light usually is not warm enough. Sunlight is best.
Why has my radiometer stopped spinning?
Not enough light or heat, or the vane has slipped off its pin. Tilt gently to reseat it and place it in direct sun.
Is it fragile?
It is thin glass. Treat it like a light bulb: fine on a shelf or a windowsill, not for a child's hands.
What size is it?
Around 15 to 18 cm tall on a glass stem. Dimensions on each product page.
Free UK delivery over £45 and 30 days to return it. For the same silent, battery-free physics, the Galileo thermometers and storm glasses.