[larrys]

I grew up in a time when microwave ovens were first released. As such the very first thing you learned is no metal in the microwave oven.

I'm wondering if people who are younger somehow think of this differently or aren't automatically taught the same thing. I mean it seems so obvious (to me) that I wouldn't even think to point it out to someone actually. It's like saying "don't let the car run over you" or "don't play catch with the laptop".

[gohrt]

This is entirely not true. When microwaves were first released, they were so low powered that microwave cookbooks gave advice like "wrap the edges of the chickin in foil".

http://www.ebay.com/itm/like/281275232630?lpid=82

Also, most microwaves have a large metal rack inside them.

Only certain configurations of metal are dangerous in a modern microwave. A computer has those, though.

http://www.todayifoundout.com/index.php/2010/08/why-you-gene...

[mikeash]

I don't think "most" microwaves have a metal rack, although they're not uncommon.

It's also not uncommon to see microwave-safe food containers that contain metal. I've seen grocery store deli soups, for example, that end up with a big ring of metal around the top when you open them, but can still be microwaved.

Still, "no metal" is a good approximation. "Unless it says you can use it" is probably OK to leave implied.

[larrys]

I've seen that as well. I'm wondering (from my own experiment) if it has something to do with angles vs. no angles [1] in terms of the metal in addition to the blocking by metal of the actual microwaves.

[1] Same as with stealth airplanes avoiding radar, right?

[ScottBurson]

You're on the right track. The strength of the electric field around a charged, curved metal surface is inversely proportional to the radius of the curve. Sharp points have a very small radius, so they generate a large electric field, which can easily ionize air and thus cause arcing.

So spoons are okay, as they normally have no sharp points. Forks can cause problems, though.

Actually there's another issue here. If you stick an unopened can of food in the microwave, the microwaves can't reach the food; you get the same effect as if you ran the oven empty, which is that the field becomes very intense and risks burning out the magnetron or other components. The oven is designed to have its output absorbed by something, not just reflected back.

[beachstartup]

microwaves induce a current in metal. if there's a gap for that current to jump, you'll see it jump, and it'll probably cause problems including fire, but if there's no gap to jump, all that voltage potential just stays in the metal.

put a table spoon in the microwave and turn it on - nothing happens. i do this all the time when reheating soup.

i've won money on small bets like this too. non-technical people have no idea WHY metal poses a problem in the microwave, so i just bet them $10 that i can put a spoon in there without problems and they never believe me. they just think i have special spoons.

[larrys]

Preface for anyone reading this: I don't have any technical degree only what I have picked up over time. That said:

So in terms of the instructions "wrap the chicken" assuming the chicken were wrapped in a way that there was no crunchiness that produced metal gaps then it would merely block the chicken from cooking in the area wrapped, right? So in theory a nice idea but in practice people slap on the tin foil and then you have sparking? Hence "no metal in microwave" is really simply not being able to rely that the general public using the product will know the nuances (which makes sense). (Human behavior is something that I do know quite a bit about..)

Maybe you've done this one: You take a glass of water in pyrex (or coffee cup) and heat it just until it is ready to boil. Then you put a spoon or other object in and it explodes. Because apparently (I think..) breaking the surface tension is the issue (which can be done with anything if you don't remove carefully and shake a bit it will also happen).

Another thing that I've noticed is that water obviously boils at different times depending on the humidity in the room (I'm non technical enough to think that I figured that one out but feel free to correct me..)

[prewett]

I think that water in a microwave can get super-heated (hotter than 100C) if the surface is smooth, or there are no more little air bubbles to start the boiling. Then, when you put something in, it gives a surface for bubbles that are desperate to form something to form on, and they all do it at once.

I had a spectacular explosion with my glass teapot one time. The microwave at work was in a different room, so I would heat up the water to boiling, and then not hear the bell, so some time later I'd remember, and do it again. Apparently reheating causes the bubbles to basically get used up so there wasn't anything to start the boiling. So the third time I was standing there waiting so I wouldn't forget, and suddenly I hear a BOOM and the pot was half empty with a lot of water outside. I think there was just one giant bubble that eventually formed and blew everything out. (The pot was unharmed)

[larrys]

"Came out" refers to when they became in widespread use. That was in the 70's.

As far as low power I can find nothing that indicates that it was ever ok to put tin foil in however I do know that in some cases you might put metal to specifically block cooking. So perhaps with a low power microwave there was no sparking etc. (I can't find anything on that and I don't see the link you sent showing that (which page is it on in the cookbook?).

http://science.howstuffworks.com/aluminum-foil-in-the-microw...

http://en.wikipedia.org/wiki/Microwave_oven#Metal_objects

http://mentalfloss.com/article/32032/why-cant-you-put-metal-...

[gohrt]

Sorry, I can't provide a proper page citation. I had one of those books many years ago, and I was surprised when I saw the recommendations about foil.

[auctiontheory]

I so 100% relate to your post that it makes me feel very old. "The kids these days" don't know about metal in microwaves?!