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100 comments on “How Does A Crookes Radiometer Work?

  1. Crookes' Radiometer is a great way to teach students and yourself the value of gathering ALL of the facts through scientific examination BEFORE jumping to wrong conclusions as Crookes and others have done.
    In order to correctly do this experiment for teaching purposes one needs the following:
    1.  "Radiometer" with an open tube at the base and a vacuuming device to allow interior gas density to be equalized with exterior gas density and decreased during experimentation.
    2. Since non-infrared "Light" has little to do with the behaviors in this experiment one should have a bright LED light radiation source as well as a dark infrared radiation source.  (Hmm, it IS radiation that causes the spin so Crooke was right!)
    3. A way to measure the temperature of the gas inside and outside the sphere simultaneously.
    4. A way to create a flow of heated air and chilled air over the top of the sphere, while increasing and decreasing the air density inside the sphere. (Don't increase it much above the exterior air density or the thin sphere will break.)
    5. Transparent Plastic wrap to isolate the thin glass semi-conductive glass sphere from outside air temperature and flow.
    6. A second Radiometer with vanes of equal color, BLACK, on both sides of the vanes.
    In this case relevant leading questions can be asked:
    1. Why do the vanes stop spinning when the gas density inside the sphere is equal to the gas density outside the tube? Does it have anything to do with density imbalance or only the inside density?
    2. At what interior gas density do the vanes spin most effectively?
    3. Why do the vanes reverse when chilled gas flow is introduced over the sphere?
    4. Why does insulating the glass sphere from the exterior air flow slow vane rotation?
    5. Introduce a small amount of smoke or other light airborne particles into the sphere to observe the eddy gas currents within the sphere while the vanes are spinning.
    This will teach goof scientific discovery skills that students will never forget.  It should also launch a branch into other areas such as;
    What causes a vortex form?
    Why does a vortex "generally" spin clockwise in the northern hemisphere and counterclockwise in the southern hemisphere?
    Why do weather patterns always spin clockwise in the northern hemisphere and counterclockwise in the southern hemisphere? "Coriolis effect"

  2. hmmm this reminds me of the classic steam engine principle of expanded gases pushing out to do work, the hot air heated strong by the dark vanes expand and as a result attempts to move to the cooler area with light vanes, as this happens, the cooler air also get displaced by the hot air entering the light vane area and ends up on the dark vane and get even hotter and this process repeats over and over assuming if the tube was 100 percent vacuum this turn effect may not work??? lets make a gigantic crookes tube and fix an alternator on its axis and produce some really powerful electric energy meaning to solar energy

  3. Just got one. This thing spins really, really fast in direct sunlight. Also tried my NiteRider 700 lumen bicycle light on it. Makes it spin like mad. The strobe mode makes it spin and look oh so cool.

  4. Maybe the Sun perceives the dark side of the metal as a witch and thus tries to burn it and that heat drives the thermal magic…so yeah…witchcraft.

  5. You have part1 of thermal creep backwards. Hot goes to cold, 2nd law of thermo. The energy is radiated from the black side to the molecules causing them to heat up and become excited. These molecules bounce around more frequently on the dark side than on the light side. This is typically referred to as pressure. Because they are bouncing more often, they are striking the surface of the fan more on the dark side than the light. It's this difference in pressure that cause the fan to spin.

  6. O̶K̶ ̶s̶o̶ ̶p̶r̶e̶s̶u̶m̶a̶b̶l̶e̶ ̶f̶r̶o̶m̶ ̶m̶e̶m̶o̶r̶y̶ ̶t̶h̶i̶s̶ ̶i̶s̶ ̶i̶n̶s̶i̶d̶e̶ ̶a̶ ̶v̶a̶c̶u̶u̶m̶ ̶s̶o̶ ̶i̶t̶ ̶c̶a̶n̶t̶ ̶b̶e̶ ̶t̶h̶e̶r̶m̶a̶l̶ ̶i̶n̶d̶u̶c̶e̶d̶ ̶m̶o̶t̶i̶o̶n̶ ̶w̶h̶i̶c̶h̶ ̶m̶e̶a̶n̶s̶ ̶i̶f̶ ̶i̶t̶ ̶i̶s̶ ̶l̶i̶g̶h̶t̶ ̶t̶h̶e̶n̶ ̶i̶f̶ ̶S̶i̶l̶v̶e̶r̶/̶w̶h̶i̶t̶e̶ ̶o̶r̶ ̶b̶r̶i̶g̶h̶t̶ ̶i̶s̶ ̶r̶e̶f̶l̶e̶c̶t̶i̶v̶e̶ ̶a̶n̶d̶ ̶B̶l̶a̶c̶k̶/̶D̶a̶r̶k̶ ̶i̶s̶ ̶a̶b̶s̶o̶r̶b̶e̶n̶t̶ ̶t̶h̶e̶n̶ ̶w̶h̶y̶ ̶d̶o̶e̶s̶ ̶i̶t̶ ̶s̶p̶i̶n̶ ̶t̶o̶w̶a̶r̶d̶s̶ ̶t̶h̶e̶ ̶s̶i̶l̶v̶e̶r̶ ̶a̶w̶a̶y̶ ̶f̶r̶o̶m̶ ̶t̶h̶e̶ ̶b̶l̶a̶c̶k̶?̶ ̶w̶o̶u̶l̶d̶n̶'̶t̶ ̶t̶h̶e̶ ̶S̶i̶l̶v̶e̶r̶ ̶a̶c̶t̶ ̶a̶s̶ ̶a̶ ̶p̶l̶a̶t̶f̶o̶r̶m̶ ̶f̶o̶r̶ ̶r̶e̶f̶l̶e̶c̶t̶i̶v̶i̶t̶y̶ ̶a̶n̶d̶ ̶m̶o̶v̶e̶ ̶a̶w̶a̶y̶ ̶f̶r̶o̶m̶ ̶t̶h̶e̶ ̶l̶i̶g̶h̶t̶ ̶s̶o̶u̶r̶c̶e̶ ̶a̶n̶d̶ ̶B̶l̶a̶c̶k̶ ̶a̶b̶s̶o̶r̶b̶ ̶a̶n̶d̶ ̶m̶o̶v̶e̶ ̶t̶o̶w̶a̶r̶d̶s̶?̶ OK Never mind that Thanks

    I also asked a number of people this but no one has an answer…

    Is there a lifespan to a photon? If we can see stars billions of light years away which means their light has been travelling that long also consider the field of view and the circumference of the sphere at that distance where you can still see the same light in any given point.

    If you has a light tight room that was full of mirrors and you set of a flash where does the light go?

    Also would Ultraviolet light work on the Radiometer?

  7. I would like to add a hypothesis. If the black side absorbs light it gets warmer. That means the molecules making up the black surface vibrate faster. Now when a gas molecule randomly touches the black surface, some of the kinetic energy of the surface is given off to the air molecule (bouncing it off, speeding it up, raising it's temperature). So it pushes the air molecule away, and the equal and opposite reaction is that the air molecule pushes just as hard on the black surface. It is like if you push someone away from you, it causes you to also move away from them.

  8. Mr frostscience, I think that your explanation is the same, the difference is only the perspective, the video was analysing the contact with the air from a thermodynamic view (when they talk about the pressure and the force of pressure that push the little pieces of papers) and what you said is just an explanation of what happen at the microscopic level.

  9. If the explanation is correct it should work poorer with less air and better with more air. There is solar wind and solar sails is there not? Why do you not present the more/air models to address this.

  10. Ok, this makes sense, but what is the practical application for this? I have memories of seeing Crookes Radiometers mounted on posts along with a couple of electrical cabinets in roadside installations in rural southern California back in the 1960s or 1970s but I can't find anything that would indicate my memories are correct, or why they would be there, or how their actions/results were recorded or used.

  11. I get it, but what's the practical purpose of a radiometer other than a toy or a demonstration device for thermal creep?

  12. You might want to take care saying Crooke didn't just cry witchcraft because he was a true scientist. While he was definitely a scientist who contributed a good deal to the scientific community, he also spent a good chunk of his life trying to prove the legitimacy of spiritualism and the ability to communicate with spirits through mediums. He actually used his radiometer to see if spirits were present. Not quite witchcraft, but not really science either. Luckily, he abandoned spiritualism later in life and got his head back in the game.

  13. So according to your explanation if I set a 45% peeled potato in a perfectly rectangular shaped mithril bowl that is made of carrots the dogs will suddeny appear and hail the unstoppable swirl of the blue puppet.

  14. thats wrong like has capacitance being that light is a coaxial circuit with dielectric pluses…. that's why if you shine a red light at it it won't spin but if you shine a violet light it will.. Violet has a higher capacity and stand read along the EM spectrum…. or if you shine a bit of sunlight on it since Sun contains everything in the Spectrum if you don't believe me watch this video….

  15. Sorry, the animation is pure bullshit. you will always see dark side on the right and light side on the left. Each leaf has a dark side (which pushes) and a light side.

  16. Reynolds found that if a porous plate is kept hotter on one side than the other, the interactions between gas molecules and the plates are such that gas will flow through from the cooler to the hotter side. The vanes of a typical Crookes radiometer are not porous, but the space past their edges behaves like the pores in Reynolds's plate. On average, the gas molecules move from the cold side toward the hot side whenever the pressure ratio is less than the square root of the (absolute) temperature ratio. The pressure difference causes the vane to move, cold (white) side forward due to the tangential force of the movement of the rarefied gas moving from the colder edge to the hotter edge. Thank you Wikipedia.

  17. Using someone else's theory doesn't mean YOU explained how it works "once and for all". Also, this theory does not take into account that red, and any color all the way to blue, light from single LED bulb won't move it while a purple light from a single LED bulb will. the white light from a bulb will contain the entire light spectrum which takes this phenomenon into account. while it's a decent theory, heat has nothing to with it. frequency of light also plays a role in this.

  18. warm air/cold air has no effect in vacuum,,,,,no air! it's all about frequencies in the uv-range and above(photons,magnetising,and several unknown forces we don't know yet),,,if i had a lab,i would try it with different metals,or even newest elements out of Mendelyev list…..thats my thinking

  19. It's all about Bernoulli effect, the dark side getting heat faster so the air near this side is hot and it expand means more pressure and it push it, and if we put it in cold refregirator let's say, the dark also getting more cold and there will be less pressure so it will be sucked and rotate the otherway around..

  20. at least bring real one, when u explain it. it's only 10$… and by the way it's not the light that makes it turn, but the heat, i tried it at home when i put ice on the glass, it starts turning the other way around.. and when its near the heat it turns this way..

  21. It has to do with the Poynting and Radiation Pressure. If you can borrow from your library An Introduction to Modern Astrophysics by Carrol and Ostlie it explains it on page 66-67.
    S(W*m^-2)= (1/u ̥ )*E x B (that is cross not multiply)
    <S> = (1/2u ̥ )*E ̥ B ̥
    F(radiation absorption) = ((<S>A)/c)*cos θ
    F (radiation reflection) = ((2<S>A)/c)*cos^2 θ

  22. You can buy this product from our site.

  23. The new Crookes radiometer by Heinsius works in another way:

  24. so its a thermal rotor . what temprarure between cold and hot would it be in thermal equilibrium. Im on the Celcius, 0 Celcius is freezing point and 100C is cooking point . is there any temprature the rotor is completely still ?

  25. The lecturer's apparent excellent explanation is ruined by the irritating, distracting music – which makes me reluctant to watch it again – which is a pity. I suggest that you please either use decent music by Bach, Mozart or Beethoven – or have none at all.

  26. How can air particles be pushing the vanes? The inside of the bulb is at a near vacuum — actually 7 torr (0.135357 psi). That means the air is spaced out with ~1 cm of separation between them inside the bulb. Hardly enough air to do much at all… if you could see the molecules you could count them!
    Still seems to me that black side of the vane is pushed by light pressure (photons transferring kinetic energy to the dark side of the vane) to the tune of about 17 micrograms per 50watts of light energy. That is a small push but the kinetic energy builds until the vane begins to move and then it builds momentum. The reflective side of the of the vane reflects the light photons and kinetic energy is not transferred to that side. I think the inventor was correct.

  27. i have this, with Philips LED bulb on the top to test for fun, hahhah. thanks.

  28. If the thermal creep explanation were correct, then the spinning would eventually stop, when the heat exchange from the radiating surface and the source of light on the light Mill reached equilibrium. This is not what we observe.

  29. I think the vanes are staying still and everything else, including us and the "flat"earth are spinning around THEM !!!!
    Yes by George me thinks me has it 😳✅🐂

  30. My father bought me one before I was a teenager (now 54). Recently I found one in Goodwill and snagged it up.

    If I remember correctly his explanation was the light was reflected off the white/silver and struck the black which absorbed it and pushed the blade.

  31. Thanks. I saw another linking momentum of light particles to such movement and it just didn't seem true to me. I suspected thermal. Thanks for the info. Cheers

  32. I have just bought one from Germany, so mine spins clockwise, do they all do this! I live in New Zealand and this may have something to do with the fact that water emptying from a sink is opposite from the northern hemisphere, or does it make a difference if the light and dark panels on the light mill are or the left or the right


  34. the colour of the plate doesn't have anything to do with the rotation you can put any colour you want it will still rotate
    the pressure inside the bulb is the raison for that rotation

  35. So it wouldnt work if there was no gas at all inside… The rotor is sucked along with the black side pulling the rotor around ?

  36. I honestly thought it was caused by light "pushing" the object. It does work.. hence the solar sails, but i dont think is that strong.

  37. Fantastic video, it inspired us to write a little app modelling the motion. Keep up the good work.

  38. If this is true then this device should turn under normal atmospheric pressure. But as I know the device don't work under atmospheric pressure

  39. Not right. Apart from what 1337wOn is saying… If the thermal theory is right then that would mean that the lightmill would spin faster in air than in partial vacuum, because even if there would be more air resistence there would also be more pressure on the black sides.. but this is not the case since it needs a certain degree of vacuum to work.

  40. if the thermal creep explanation was correct ….then red light would also have turned it cause it gives more infrared signature …but it doesn't … only purple and white light does …check ur facts

  41. Common thinking might suggest that black absorbed and white reflects. Just like a magnet.
    In fact it is the opposite. Black reflects and white absorbs.
    The Crooks unit exemplifies one of seven laws of nature.

  42. Wonderful explanation. Finally understood the concept derived by MR Reynold. Do u know the theoretical background of thermal creep or where to find it? Would really help with my research on this! Thank you.

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