[HN Gopher] Crookes Radiometer
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Crookes Radiometer
Author : niklasbuschmann
Score : 65 points
Date : 2024-03-20 20:56 UTC (1 days ago)
(HTM) web link (en.wikipedia.org)
(TXT) w3m dump (en.wikipedia.org)
| volemo wrote:
| Interesting videos that explain how the radiometer works:
|
| - https://youtu.be/t-JN2U4jHgk by Applied Science
|
| - https://youtu.be/r7NEI_C9Yh0 by Technology Connections
| erk__ wrote:
| And one that says a bit about Sir William Crookes and shows of
| some of the Radiometers he made:
|
| - https://www.youtube.com/watch?v=p0f5i7r02dg by Objectivity
| jihadjihad wrote:
| > [Crookes] was weighing samples in a partially evacuated chamber
| to reduce the effect of air currents, and noticed the weighings
| were disturbed when sunlight shone on the balance. Investigating
| this effect, he created the device named after him.
|
| Whenever I read about little discoveries like this, I get a
| smile. Same thing with the guy that had the candy bar in his
| pocket and discovered microwave energy could be used to heat
| stuff up...must have been an exciting day!
| dekhn wrote:
| chance prepares the favored mind
| dspillett wrote:
| _> discovered microwave energy could be used to heat stuff_
|
| It was already well known that EM waves / photons could heat
| things, sunlight to give the most obvious example.
|
| His discovery was how effectively specific frequency ranges1
| can be at heating food by exciting the water within -
| effectively enough that the technique once refined would be
| useful for commercial and even domestic cooking, safely and at
| practical cost.
|
| --
|
| [1] domestic ovens use +-2.45GHz, 915MHz is also effective and
| used in larger commercial ovens as it can penetrate deeper into
| the material being heated
| CamperBob2 wrote:
| 2.45 GHz has nothing to do with the physics of water
| molecules or anything else. You could do the same thing at 13
| MHz (diathermy) or 95 GHz (active denial). 2.45 GHz is just
| where the FCC dumps RF signals they don't want to deal with.
| Turing_Machine wrote:
| You're correct that other frequencies would work, but have
| the causality backward when it comes to 2.4-2.5 GHz being a
| garbage dump. The nascent microwave oven industry was
| making it into a de facto garbage dump, so the FCC
| requested that the ITU assign it as an official garbage
| dump. In other words, the ovens weren't put there because
| it was a garbage dump, it became a garbage dump because
| they were already there.
| CamperBob2 wrote:
| Yeah, good point there. Few licensed services probably
| wanted to co-exist with the RF leakage from a microwave
| oven.
|
| I imagine that 2.45 GHz was chosen as a compromise
| between cavity size/cost and food-penetrating power, but
| I don't think I've seen a definitive historical answer.
| The sweet spot for cooking effectiveness might even be at
| a somewhat-higher frequency, but it would have been more
| expensive to build ovens at higher frequencies back in
| the day, and it would also be more expensive to shield
| them properly. Like the GP suggests, at lower frequencies
| the size of the whole thing starts to become a problem
| for household use.
| jhoechtl wrote:
| No, the dimensions of the magnetron changes and the mesh
| size at the door, otherwise it's just the same.
|
| Penetration depth at higher frequencies is lower, a lower
| resonant frequency penetrating deeper the tissue while
| engaging water molecules would be better
| Turing_Machine wrote:
| This is pure speculation, but I wonder if it might also
| have something to do with surplus cavity magnetrons (and
| the production facilities for them) becoming available
| post-WWII. I can't find anything definitive on production
| models, but I was able to determine that the prototype
| magnetron that Churchill sent to the United States in
| secret operated at around 3 GHz...pretty close. Maybe
| cheaper to use the ones that already had production lines
| set up, if they were otherwise workable?
| dtgriscom wrote:
| https://en.wikipedia.org/wiki/Peryton
| AnimalMuppet wrote:
| What does "active denial" mean in this context?
| CamperBob2 wrote:
| At higher frequencies, the problem with 2.4-GHz microwave
| ovens only tending to cook the outer layers of food
| becomes even more acute. If you were to actually build a
| microwave oven at 95 GHz, it would suck because only the
| very outermost layers of the 'food' would see any
| meaningful heating.
|
| Raytheon turned that particular lemon into lemonade:
| https://en.wikipedia.org/wiki/Active_Denial_System
| fullspectrumdev wrote:
| I have a couple of these literally as household decoration. They
| are really quite neat.
| weberer wrote:
| Same, I found a 10 pack for $20 online a decade ago, and just
| lined them around the house.
| tomsmeding wrote:
| > The Reynolds paper went unpublished for a while because it was
| refereed by Maxwell, who then published a paper of his own, which
| contained a critique of the mathematics in Reynolds's unpublished
| paper. Maxwell died that year and the Royal Society refused to
| publish Reynolds's critique of Maxwell's rebuttal to Reynolds's
| unpublished paper, as it was felt that this would be an
| inappropriate argument when one of the people involved had
| already died.
|
| Modern double-blind reviewing processes certainly have
| advantages.
| peterburkimsher wrote:
| My dad had one of these at his desk at CERN, and would challenge
| visitors to his office to explain the motion.
|
| I have one at my desk now, and unfortunately it doesn't move. I
| wish I sat closer to a window.
| anfractuosity wrote:
| Just came across this video, mentioned on the wikipedia page -
| "Nano-sized light mill drives micro-sized disk"
|
| https://www.youtube.com/watch?v=oh5DPC1zoUs
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