The Radiometer

The history of the Crookes Radiometer and some experiments that may be performed with a modified commercial unit.

This original version of this article appeared in Volume 4, Number 1 of the Bell Jar.

The first paragraph is drawn from material developed by Franklin B. Lee in “Experiments in High Vacuum” (Morris and Lee, Buffalo, NY, ca. 1960) and is used with permission. It must be noted that Franklin’s explanation of the operation of the radiometer, while the popular explanation, is not correct. For a full explanation, please refer to Philip Gibbs’ article “How does a light-mill work?” In progress is an article that will cover thermal transpiration (the real cause of the vanes’ turning) and how this phenomenon is applied to gauging and pumps. - Ed.

A radiometer consists of a set of vanes, each shiny on one side and blackened on the other, that is mounted in an evacuated vessel. When exposed to light, the vanes revolve. The first radiometer was constructed to settle the controversy regarding whether light exerts a force. The idea was that a reflecting surface would experience a greater force from the light than an absorbing one. The instrument was therefore made in the now familiar form. Unexpectedly, the opposite effect was observed. The blackened vane retreated from the light source. We now know that the black surface is warmer than the shiny one and that gas molecules will recoil faster from the hot surface. The slight difference in molecule recoil is what causes the device to spin. (Later experiments in a much better vacuum have confirmed that light does exert a very small pressure.) The action of the radiometer depends upon striking a balance between molecular drag and recoil. At higher pressures, drag will dominate and the vanes will fail to spin. At lower pressures, there are too few recoiling molecules to drive the vanes. The optimum balance occurs at a pressure of about 60 mTorr (60 microns Hg). By using a suitable tachometer (e.g. a ‘Strobotach’ or an electronic counter with a photocell that detects the interruption of a light beam by the vanes) it would be possible to measure the change in rotational velocity with changing pressure, given a constant light input.

While the radiometer is not a very good gauge in itself, Dushman in “The Scientific Foundations of Vacuum Technique” noted the use of the radiometer to determine when the vacuum in an incandescent lamp had reached the required level for sealing-off. At the proper pressure, the vanes would cease to rotate, even in very bright illumination (this would be on the low side of the ~60 mTorr peak). He also noted that the level of vacuum could be quantitatively determined by shaking the bulb to set the vanes in motion and then noting the rate at which the spinning ceases. This notion has been embodied in the modern spinning rotor (molecular drag) gauge.

Ready-made radiometers are available from science supply houses. Also they are increasingly popular as window ornaments and can often be obtained for about $10 from local craft shops. I've also seen radiometers in the windows of “New Age” boutiques, leading me to wonder what strange powers people might attribute to them.

For tinkerers, the disadvantage of ready-made radiometers is that they are sealed off. Fortunately, the glass pump-out tube is readily accessible. With a file, nick the end of the pump-out and break the tip off. (I’d suggest placing a piece of rubber or vinyl tubing over the glass to prevent cuts). Using epoxy cement, seal a length of 5/16" OD brass tubing (K&S Engineering, available at well stocked hobby and hardware stores) to the bulb. Be careful not to get epoxy into the original evacuation ports in the stem. The hanging style of bulb is the most convenient to use.

This tube may be attached to a blank brass KF flange. Drill a 5/16" diameter hole in the flange and solder the tube in the hole. The flanged radiometer may now be attached to a vacuum system that is capable of evacuating to a few tens of milliTorr. (See Figure 1.) Using the pressure control feature, a pressure vs. rotational speed plot may be constructed.

If no local source can be found for the radiometer, surplus units have usually been available from American Science and Surplus.

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