Crookes Radiometer

2 Articles

A circular metal vessel is shown, with a symmetrical rotor of four vanes standing inside. At the bottom of the vessel are four loudspeakers.

Building An Acoustic Radiometer

November 22, 2025 by 12 Comments

A Crookes radiometer, despite what many explanations claim, does not work because of radiation pressure. When light strikes the vanes inside the near-vacuum chamber, it heats the vanes, which then impart some extra energy to gas molecules bouncing off of them, causing the vanes to be pushed in the opposite direction. On the other hand, however, it is possible to build a radiometer that spins because of radiation pressure differences, but it’s easier to use acoustic radiation than light.

[Ben Krasnow] built two sets of vanes out of laser-cut aluminium with sound-absorbing foam attached to one side, and mounted the vanes around a jewel bearing taken from an analog voltmeter. He positioned the rotor above four speakers in an acoustically well-sealed chamber, then played 130-decibel white noise on the speakers. The aluminium side of the vanes, which reflected more sound, experienced more pressure than the foam side, causing them to spin. [Ben] tested both sets of vanes, which had the foam mounted on opposite sides, and they spun in opposite directions, which suggests that the pressure difference really was causing them to spin, and not some acoustic streaming effect.

The process of creating such loud sounds burned out a number of speakers, so to prevent this, [Ben] monitored the temperature of a speaker coil at varying amounts of power. He realized that the resistance of the coil increased as it heated up, so by measuring its resistance, he could calculate the coil’s temperature and keep it from getting too hot. [Ben] also tested the radiometer’s performance when the chamber contained other gasses, including hydrogen, helium, carbon dioxide, and sulfur hexafluoride, but none worked as well as air did. It’s a bit counterintuitive that none of these widely-varying gasses worked better than air did, but it makes sense when one considers that speakers are designed to efficiently transfer energy to air.

It’s far from an efficient way to convert electrical power into motion, but we’ve also seen several engines powered by acoustic resonance. If you’d like to hear more about the original Crookes radiometers, [Ben]’s also explained those before.

[Ben Krasnow] Shows Us How A Crookes Radiometer Works

November 15, 2014 by 21 Comments

[Ben Krasnow] is tackling the curious Crookes Radiometer on his Applied Science YouTube channel. The Crookes Radiometer, a staple of museum gift shops everywhere, is a rather simple device. A rotor with black and white vanes rotates on the head of a needle. The entire assembly is inside a glass envelope. The area inside the glass is not at a hard vacuum, nor is it filled with some strange gas. The radiometer only works when there is a partial vacuum inside.

The radiometer’s method of operation was long misunderstood. Sir William Crookes and James Clerk Maxwell both believed that the vanes moved due to the pressure of the photons hitting the vanes. If that were true though, the radiometer would spin in the opposite direction it normally does when held near a light source.  It was eventually discovered that the system is a thermodynamic one. [Ben] proves this by cooling down the radiometer’s glass with a can of freeze spray.  The radiometer immediately begins spinning backwards, with no light source present.

From there [Ben] mounts the rotor of a radiometer inside his vacuum chamber, which many will recognize as the chamber from his DIY electron microscope. As expected, the vanes don’t spin at a hard vacuum. In fact, [Ben] find the vanes spin fastest when the pressure is about 7 mTorr.

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