Properties of the Radiometer Force

L. Thompson

Extensive literature is on the rarefied gas reactions of the radiometer. Yet, little attention seems given to the actual properties of the radiometer force. The following testing is to show some of these properties.

In the radiometer the application of a force overcomes the inertia of the vanes and harness. In atmospheres above 1000 +/- microns the crowding effects of the gas molecules prevent movement. In high vacuum pressure differentials are not present as the rarefied gas spacing is far apart for pressure differentials and for slip effects.

The testing is in the partial vacuum range of  100 to 300 microns. In this range the radiometer vanes and harness, placed in the vacuum chamber as a control check, show a similar response as in a sealed radiometer.

Many variables can increase or decrease the force/power. Some variables could be energy input, atomic weights of the gases, vane spacing, shape and type of the surfaces, and the degree of vacuum. 

Yet in the complexities is a simplicity. This simplicity is: the radiometer vanes rotate in response to the input of energy or to the taking out of energy.  In the testing  I use this simplicity to show the characteristics of the resulting force. I call this force a composite net force because the transfer of molecular momentum also results in the deceleration (cooling) of the individual rarefied gas molecules to give the net acceleration of the vanes and harness.

Observations and Testing Notes

1. In a radiometer the net force results in centrifugal acceleration - as if the force needs to "push itself" into a lower pressure area. 

2. When the rarefied gas molecules transfer momentum, the gas molecules are cooled - are decelerated. This cooling, simultaneously taking place with the transfer of molecular momentum into the macro momentum of the vanes, would represent the "exhaust side" of the generated force/power.

3. After breaking the radiometer sealed tube to let in air/oxygen, a strong heat source (60w halogen)  was applied to the vanes.  Vane combustion occurred.

Before breaking the sealing tube, the same energy source had been applied. The vanes spun rapidly.  The spinning radiometer vanes, with the input of molecular energy to vane momentum, would then then be running cooler. There was also more time in the rotation itself to dissipate any excess heat as radiated energy. 

4. In the testing the gas molecules are recharged and reused without external exhaust. With the continual input of energy, the gas molecules are trapped to do work  and  to be reused.

5. With a strong halogen light , the vanes first spin clockwise then upon cooling spin opposite, counter clockwise. Radiant energy would now be radiating at a faster rate from the carbon black surface, creating an opposite pressure differential.  (This would be the refrigeration effect when a radiometer is taken out of a refrigerator.) 

6. I feel the initial energizing of the gas molecules could involve Newtonian physics as the molecules would be striking the irregular carbon black surface from various angles and over short time intervals. The individual gas molecules also could not overcome the larger inertia of the vanes. If some "back force" would be exerted on the non reflective side it would  be less than the gas molecules acting in mass via the slip effects at the vane edges. 

The end result could be a force with little or no initial equal and opposite offsetting reactions and seemingly counter to Newtonian physics. Yet this net macro force could have a base in Newton's physics - that a small potential individual molecular force, as in a bouncing ball, would not overcome the inertia of a larger mass.  Only the direction of momentum would be changed, not the potential energy/force.

7. My understanding of radiant energy is little or no force would be exerted on the radiating mass. Hence it could be possible to also create an imbalance when excess energy would be re-radiated back out of the system (as the refrigeration effect) to cause the vanes to then spin oppositely.

8. I confirmed with a light weight carbonized aluminum strip hanging with an off center hole on a carbon fiber rod that it was possible to move the strip down the rod.  Extrapolating the weight from a larger aluminum strip the weight was about 0.002 grams. The strip moved first with agitation and then rapidly with full circular motions around and down the rod. 

I am now almost certain that oscillations are necessary (from high to low pressure areas) to commence movement. This helps explain why I was frustrated in the testing in not being able to have heavier strips move along a rod - though a little back and forth movement took place. It helps explain why the much weightier control mounted radiometer vanes on the needle pivot spin easily whereas aluminum/carbonized strips mounted on a rod showed little motion due to "unbalanced friction" preventing oscillations.  

Yet, if needing further confirmation of the necessity for oscillations, the small, carbonized, very light aluminum strip with its off center hole spun energetically around the rod as it moved down the rod.  The degree of energizing looked to match the radiometer vanes upon the application of a strong energy source.

Although this test requires a very small mass, I also observe the slip effects could be made to be "channeled" into a directional movement along the rod. 

In summarizing point 8, slip effects can be well enhanced when centrifugal acceleration is included to also enhance the pressures differentials. This could help explain why the well balanced, much weightier radiometer vanes, on the needle pivot can reach 2000 to 3000 rpm.

A Future Test

A future test of these special properties could be as follows. Fix a set of elongated vanes inside an elongated light weight glass "bulb" (similar to a closed cylinder) using the high strength glass now available. The surface area could be say 5X times that of the typical hobby radiometer. Leave sufficient space around the vanes for the rarefied gas reactions and slip effects. A small opening is left in the "bulb" to equalize the rarefied gas pressure with the surrounding vacuum chamber. Instead of the vanes pivoting on an interior needle, the entire "bulb" is to be well balanced and mounted on pivot needles on the top and the bottom. 

Although this apparatus is X times weightier than the vanes and harness of a radiometer, I have retained the centrifugal nature. Some offsetting weight considerations are: the low exterior drag of the vacuum chamber "exterior" atmosphere compared to the exterior radiometer full atmosphere  and the 5X +/- increased surface area. 

If sufficient power can be developed it may be possible to run this test outside of a vacuum chamber and under a full external atmosphere. 

By itself this test would be of little significance, a toy, a demonstration similar to a person walking in an enclosed cylinder showing that an internal force can cause the acceleration of the total mass. 

Yet what I am "getting at" in this test could be the significance: the interplay of the variables and the composite nature of the force with little or no initial offsetting force.  


A. Testing is to be under the supervision of those with experience in vacuum safety. 

B. The deceleration of the rarefied gas molecules is "masked" by the macro acceleration of the vanes and harness. 

C. Newton's physics is focused on the taking apart and quantifying force. I feel the radiometer force, in contrast, is a composite force. This is why a fuller explanation of the radiometer could require  looking at the radiometer as a total accelerative process - including the deceleration of the rarefied gas molecules.


The thermal explanation in this reference could be misleading. The rarified gas molecules become cooled when energy is taken out of the gas molecules in the transfer of energy to the vane rotation. There is thus no need, under ordinary conditions, for external cooling of the radiometer.  Once vane rotation equilibrium is reached, radiant cooling could be sufficient to transfer excess heat from the system.

E. I would like the proposed test run in a qualified physics laboratory. Perhaps other perspectives could come from the testing of these special case properties which have little or no initial offsetting force.

Revised 7/17/2014.