How to Use a Tabletop Radiometer to Show Sunlight Energy

By Art of Play - Playing Cards, Puzzles and Amusements | Published: 2026-11-01

Category: How-to Guides

Set up a tabletop radiometer for a clear sunlight demo. Learn how a Crookes radiometer spins, what light sources work, and the mistakes that stop the vanes.

Quick answer

Place the radiometer in direct sunlight or strong artificial light and watch the vanes spin. The darker sides absorb more light energy, and the temperature difference drives the rotation. Keep the bulb still and use a bright, unobstructed light source for the clearest demonstration.

Why the vanes spin, and why the bulb matters

A Crookes radiometer is a glass bulb with four two-tone vanes on a central spindle. The air has been removed to create a partial vacuum, so the vanes can rotate freely. When light hits the vanes, the darker surfaces absorb more energy than the lighter ones. That difference creates a tiny temperature imbalance, and the vanes turn.

The partial vacuum is essential. Too much air inside would damp the motion, and too little would change the effect. This is why you should not open, heat, or press on the bulb. Treat it as a sealed instrument, not a toy to take apart.

Choosing a light source for the demo

Direct sunlight gives the most reliable spin. A sunny windowsill or an outdoor table works well. Artificial light can also work, but it must be bright enough and aimed at the vanes. A desk lamp held close may produce a visible response, while dim room lighting often will not.

If the vanes barely move, the light is probably too weak or too diffuse. Move the radiometer closer to the window, or use a stronger lamp. Avoid placing it behind glass that blocks too much light, and do not expect a response in a dark room.

Step-by-step radiometer sunlight demo

Place the radiometer on a stable surface: Set the base on a level table or windowsill where it will not be bumped. The vanes need to spin freely on their spindle, so a wobbling surface can stop the motion. Check that the bulb sits upright and does not rock.
Aim it at direct sunlight: Turn the bulb so the vanes face the sun, or move it to the brightest part of a sunny window. Avoid shade from curtains, frames, or nearby objects. If the vanes do not move within a minute, move the radiometer closer to the glass or outside.
Try a strong artificial light if sunlight is unavailable: Hold a bright desk lamp close to the bulb and aim it at the vanes. Keep the lamp steady and wait a moment for the vanes to respond. Dim or distant lamps may not provide enough light; use the brightest source you have.
Observe the direction and speed of the spin: Watch how the darker sides of the vanes lead the rotation. The spin should be smooth and continuous in steady light. If it stutters, check for shadows or vibrations. A sudden stop usually means the light moved or something is blocking the bulb.
Keep the bulb sealed and handle it gently: Do not open the bulb, press on the glass, or expose it to heat. The partial vacuum is part of the design, and damage will stop the vanes from turning. Store it away from edges and out of reach of young children.

Product facts

Tabletop Radiometer: The Tabletop Radiometer is a glass bulb with four two-tone vanes on a central spindle, and the air has been removed to create a partial vacuum. That sealed vacuum is what lets the vanes rotate freely, so the demo depends on keeping the bulb intact and still.
Tabletop Radiometer