Why doesn't the metal ring on Campbell's Soup at Hand cause sparks in the Microwave?

The instructions say to leave the plastic lid off when heating, and sure enough it’s not a problem. Why is that?

Huh? The instructions on MY Campbell’s says to leave the perforated plastic lid on when microwaving. Though I’m also perplexed as to how the metal rim is safely microwaveable:)

Charge tends to accumulate on the sharp edges of the metal, so this is where your corona discharges (sparks) are going to occur. Anything smooth, like the campbell’s ring, a well rounded spoon, or a ball bearing, isn’t going to arc. A fork on the other hand, since it has lots of nice sharp edges, makes a much more interesting display.

There are 2 different kinds of Campbells soup that microwave in the container. You have the drinkable Soup at Hand and the Microwavable Bowl. The microwavable bowl version has you keep the perforated plastic top on because it will boil over onto the side if you don’t.

If you don’t mind a hijack, since ECG has pretty much answered the question, has anyone tried the microwaveable bowl clam chowder? I did, and the bowl did a whole ‘mexican jumping bean’ think while heating up. I was afraid it was going to explode!

Yes, but there’s more to it than just that. There also has to be some place for the charge to jump to through the air to produce arcing. There aren’t any gaps in the metal ring, so even though a current is being induced in it, no sparks are produced. Cut a gap in it and watch the fireworks. The ring is also to far from the metal walls of the cavity to arc over.

but the bowl of Chicken Noodle does the same thing. Freaked me out as I was sure it was the metal ring (the first time) and the office microwave.

Another important factor is the amount of liquid present. Water absorbs microwave energy very effectively, and a large enough quantity can soak up enough energy that even a fork won’t spark (although I don’t recommend trying this at home). The proximity of the metal to the soup thus reduces the tendency to produce sparks.

To summarize:

  1. No spark gap of appropriate dimensions (although with a sufficiently intense field, you could theoretically get an arc from one side of the ring to the other, assuming the ring isn’t perfectly round).

  2. No sharp/pointy bits to accumulate charge.

  3. Damping effect (if you’ll pardon the expression) from proximity to the soup.

Huh! I routinely microwave forks or spoons with no problem. The only time I’ve seen fireworks, was when I put something in that had a bit of tinfoil on it. THAT was impressive!

Nope, not true for high voltage RF. I’ve attempted heating canned foods in microwaves, and if there’s even a tiny burr on the rim of the can, it usually spawns a huge “blow torch” corona discharge that makes a brilliant blue light and a loud 120Hz buzz. It’s definitely NOT from broken circuit-rings, since single pieces of metal do this too. I didn’t see it as often in my old 500 watt oven, but the 1000W oven does it reliably. This is similar to the brush discharge at the top of a small Tesla Coil, but in a microwave oven the power level approaches a kilowatt, so the “corona discharge” is anything but feeble.

One fun thing I’ve tried is sticking a wooden toothpick in a cracker or some other support structure to keep it upright, then lighting it on fire and putting it in the microwave. After a few seconds, you get wonderful violet-blue fireballs rising up off the flame. Oddly, though, it doesn’t seem to work with a plastic toothpick or a candle. Anybody know why this happens? What is it about a little flame, or a burning toothpick in particular, that creates these delightfully unnerving balls of electricity?

We had a book on cooking with your microwave (circa 1980s of course) and it claimed that you could microwave metal safely- as long as the metal part of the packaging accounted for no more than 10% of the packaging’s mass. Never having tested this theory (no scale, ha!), I wonder if it’s correct. The fact that chef boyardee stuff has come with metal rings in its packaging for several years makes me a little less wary of the possiblity, though.

I wouldn’t post this, but the OP has been pretty well answered.
We tried to dry a smoldering joint once, and got the same effect. The smoke really went off.

chorpler: The reason a wooden toothpick works and not a plastic one is because the rising balls of flame are actually bits of burning carbon. The plastic provides no such “seed” for the plasma formation.

So, the arcing mentioned in this thread do not harm a nuker?

So, the arcing and fireballs mentioned in this thread won’t harm a nuker?

I’d like to ty some of this stuff, without worrying that I’ll have to buy a new microwave.

Wait until January. The stores often clear out decent-powered microwave ovens after Christmas rush for $30-40 (more or less)

Just what the doctor ordered. A smart ass who thus far has contributed nothing to this discussion, but then comes on with a gratuitous remark.

Um, that was kind of mean, Antiochus.
Our office microwave has a wire shelf in the middle, like an oven rack.

It’s disconcerting after so many years of being told that metal=no.

Gravity - take the rack out and bend it pretty well. That make life interesting.

At the risk of being seen as someone who posts gratuitous remarks, I’ll add that if you don’t want to ruin the office nuke, you can pick up some cheap microwaves, plug them in in the garage, and play scientist.

There wasn’t anything gratuitous about it. You said you were worried about ruining yur oven, and he told you how to get one cheap. How is that gratuitous exactly?