> [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!
> 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.
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[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
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.
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.
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.
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?
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
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.
Perhaps there's a way to visualize the actual operation of the Crookes radiometer, I've a proposal below but I'd welcome a better one.
As with many of us, I've been fascinated with the radiometer's seeming 'magic' operation since I was a young kid and first saw one in a shop window. Back then and when I was doing high school science we were led to believe that radiation pressure from incoming light alone was its modus operandi and none of us questioned otherwise. When I learned later of the now accepted Reynolds explanation I was somewhat surprised that we weren't taught it at school as it was already known (seems too our science teachers were unaware of it, and for some reason they never questioned the anomalous direction of rotation, nor did we think to ask).
I admit I've always been surprised that radiation pressure was strong enough to cause rotation because it just seemed that overcoming inertia alone would be a problem which would mean the vanes would have taken a much longer time to spin up than they do in practice. Furthermore the frictional losses in the glass/pivot bearing would have had to have been incredibly small. BTW, I'm not taking credit for smart thinking here as others I'd discussed it with thought similarly. When we eventually learned of the Reynolds explanation it made sense.
it's just occurred to me—and I haven't thought this through with any finesse—but perhaps there's a way to visualize the radiometer's operation by making the movement of the residual low pressure air visible by some means, but that seems quite a challenge.
Coloring the air wouldn't work but there might be another way. Suppose the air was swopped with say radon gas or perhaps even uranium hexafluoride (although I'd suspect it would be much less efficient) and we cover the inside of the radiometer's bulb with numerous radiation detectors but not enough to block sufficient incoming light to stop it functioning. (The detectors would have to be on the inside of the bulb as its glass would block the alpha particles emitted from the radon).
We could then track density changes in the gas and build up a moving image of its movement versus incoming radiation levels. Little doubt it'd be a difficult to do and likely considered too dangerous and frivolous an experiment to make it worthwhile. Nevertheless it'd be fascinating to see visual images of the gas's movement, also they'd put any remaining arguments about the radiometer's operation to rest.
Anyone got any suggestions for better ways of making the gas molecules visible?
> 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.
- https://youtu.be/t-JN2U4jHgk by Applied Science
- https://youtu.be/r7NEI_C9Yh0 by Technology Connections