All spring, summer and fall, it's been the same: Charlie's refrigerated formula does 33 seconds in the microwave for four ounces, 44 for six, and a minute for eight.
Until last week. Julie put eight ounces in the bottle, microwaved it for a minute, and brought it up to me. I gave it to Charlie, who, after a few sips, indicated, "No, thanks, that's really a bit too warm, AND I'M STILL VERY THIRSTY WHERE'S MY MILK!" (Testing with a finger proved the first half of his commentary correct; the second was of course true by definition.)
I thought it must have just been one of those mistakes you make with not-completely-chilled bottle of formula, but then the other day I nuked Charlie's milk just the same way, and darned if it wasn't too hot again.
The only thing I can think of is that with the coming of winter, the ambient air has gotten drier and stopped absorbing quite so many microwaves as it cycles through the nuke. We're talking a difference of something like 10-20 grams of water vapor per cubic meter (the oven itself it nowhere near that size, but I think the fan runs air through it), which could be just enough to tip the balance from comfortably warm to too darn hot. If so, it would be a remarkable example of physics in action.
Just to prove that water vapor and not just liquid water does absorb microwaves, I invoke my physics and public policy professor's story about the first US-designed magnetron radar tube during the early days of Word War II. The damn thing didn't work and didn't work and didn't work, but in an odd way: they could measure the power going in, and it was what they expected, and they could measure the power coming out, as expected too. But it just didn't produce any radar returns for targets more than a few hundred yards away.
The problem, he explained, was that someone (perhaps subconsciously using one of the few microwave frequency numbers they knew) had designed the tube to radiate dead in the middle of the primary absorption band for water vapor. It was a drizzly cold fall in the northeast, and QED.
Posted by wallich at November 19, 2005 08:29 PM