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The Straight Dope



Why does metal catch on fire in the microwave? --Maura McCormick, Rockville, Maryland

Can't say I've come across any cases of metal actually catching on fire in a microwave, though my experiments in this respect haven't been as extensive as I'd like, owing to Mrs. Adams's refusal to sacrifice (potentially) the household microwave to the cause of science. What you do see is sparks, which may pit the metal. You get sparks because the intense microwave radiation generates a fluctuating current in the metal, a process known as induction. Due to internal resistance, at any given instant there are sharp differences in electrical potential between different points on the metal surface. Because the microwave is pumping in a lot of energy, these differences in potential increase to the point that a spark jumps the gap. Basically tiny lightning bolts, the sparks are very hot and I suppose could set fire to, say, a nearby paper plate. The metal may also become very hot and glow, due to incandescence, or even vaporize. But worse things could happen. The metal surface may reflect the microwaves back where they came from, namely the oven's magnetron tube. That could damage or destroy the tube, which is why manufacturers tell you not to put metal in your microwave.

All of which is a long-winded way of saying you should NOT try this at home. But if you do anyway (heh, heh), you might let me know what happens. A couple spuds who obviously got turned down for one Friday-night date too many have written to say microwaving a filter cigarette will cause flame to shoot out the filter end. Sounds cool, but unfortunately I haven't been able to replicate the phenomenon. Mrs. Adams again. Thank God she wasn't married to Edison or we'd all be sitting here in the dark.


In a recent column you mentioned the problem of crossing borders between two countries that drive on opposite sides of the road [August 12]. I too have wondered about this. At first I thought there were no such borders since the only countries that drive on the left are island nations such as the UK, Ireland, and Japan. But then I found out that's not true--many continental nations formerly ruled by Britain, such as India, still drive on the left. But I found the answer in the enclosed article from the Internet. --Robert Teeter, San Jose, California

Cecil didn't answer this question when it first came up because he thought it was too dumb, and while he appreciates Robert Teeter's efforts, on reading the enclosed article he's gratified to discover that it was too dumb. I quote from "Which side of the road do they drive on in...?," available via ftp (Netters know what I'm talking about, and everybody else can rest assured they're not missing much) from

"BORDER CROSSINGS.... This is not such a great puzzle as it might seem. Here are a few stories from people who have accomplished this mystifying feat.

"'It was not a problem at the only border I have been to like this (Zaire to Uganda). The traffic was slow and there was very little of it. There was just a sign reminding you to swap sides.'

"'The border crossing from China (where they drive on the right) to Pakistan (where they drive on the left) merely has a sign at the side of the road that says "Entering Pakistan, Drive Left" and for those going the other way "Entering China, Drive Right."'

"'Usually you don't drive straight through a border post. The only place I've crossed a land border where the side of the road for driving changes is between Afghanistan and Pakistan. We drove into a car park (using the right hand side) and after the border formalities, drove out using the left hand side.'"

So there you have it: they put up a sign telling you to change sides. Who would have thought it? Me, for starters. But for those who found this a real stumper, I'm glad we got things cleared up at last.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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