Thermal Conductivity for Dummies

You'd like to think... well, we'd like to think that if you ask someone how device A works, you'd get some sort of discussion of the principles behind its operation, even if the person asking is a ten-year-old. You'd especially like to think that a teacher would be able to get it across; but that's not the case with eHowian Janet Beal. Janet did a laughably poor job of answering the question, "How Does a Thermos Work?¹" for the mother site (eHow.com). Why it was moved to HomeSteady.com, we have no idea...

Whatever the case, Beal dumped out just under 300 words, two-thirds of which were devoted to the sub-headings "And then the thermos broke!" and "And the lemonade was still cold!" That's right: despite claiming an "MS in early childhood education," the best Janet could come up with for explaining how a thermos works is,

"A thermos jug or bottle works on a very simple principle--the insulating property of air. A thermos bottle has two walls between which there is an empty space. Both the inside and outside walls are usually made of materials that do not transfer heat well. Thus, the hot or cold contents of the thermos bottle stay hot or cold while the outer layer of the bottle and the air between the two layers slow down heat transfer either in or out of the bottle."

Too bad the notion that the inner and outer walls "do not transfer heat well" is utter crap! A thermos works because whatever is between the walls has a low thermal conductivity. It's sometimes air, and sometimes a vacuum. The heat-transfer properties of the inner and outer walls are essentially immaterial. Still, some of our staffers remember that the glass liner of their childhood bottles was silver. Beal neglected to mention this; a feature that further decreased heat transmission through the liner by reducing radiant heat transfer.

Instead of expanding on the insulating properties of air and vacuum or attempting to explain why air is a poor conductor of heat (the gas molecules are randomly arranged and relatively far apart), Beal goes on to babble about breaking the glass in her thermos bottle back in the '50s, and finished by asking, "What would a lunch box be without a thermos?" She didn't even mention vacuum!

What, indeed... apparently our Dumbass of the Day hadn't looked into a lunch box in a couple of generations...

¹ The original has been deleted by Leaf Group, but can still be accessed using the Wayback machine at archive.org. Its URL was   ehow.com/how-does_4568274_a-thermos-work.html

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