# How do thermoses know if they are supposed to keep drinks hot or cold? **URL:** \ **Category:** Factual Questions\ **Created:** [August 23, 2010, 12:05am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114 "2010-08-23T00:05:05Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![ramel](https://avatars.discourse-cdn.com/v4/letter/r/eb8c5e/32.png) [@ramel](https://boards.straightdope.com/u/ramel)\ **Post date:** [August 23, 2010, 8:04am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/21 "2010-08-23T08:04:56Z")
They slow the movement of heat. In or out. So, they slow the heat leaving something hot, or they slow heat entering something cool. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [August 23, 2010, 9:35am UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/22 "2010-08-23T09:35:41Z")
My thermos has a switch on the side to choose between keeping things hot of cold. It works great and only cost $49.05 + shipping and handling from the shopping network. It never needs batteries either which is a huge bonus. I have no idea how it works though. Technology is an amazing thing. ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [August 23, 2010, 12:14pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/23 "2010-08-23T12:14:02Z")
> [@Shagnasty](#): > > My thermos has a switch on the side to choose between keeping things hot of cold. It works great and only cost $49.05 + shipping and handling from the shopping network. It never needs batteries either which is a huge bonus. I have no idea how it works though. Technology is an amazing thing. I have a battery-powered version. The amazing thing is that it still works when the batteries have died. ---
**Author:** ![muldoonthief](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muldoonthief/32/3188_2.png) [@muldoonthief](https://boards.straightdope.com/u/muldoonthief)\ **Post date:** [August 23, 2010, 1:56pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/24 "2010-08-23T13:56:32Z")
> [@Chronos](#): > > > [@](#): > > > > If you were to consider him the fourth, whom would you consider the third? > > Newton and Einstein are the top two, then maybe Archimedes, then Maxwell. How about Faraday? Experimentalists need love too, you know. ---
**Author:** ![JoelUpchurch](https://avatars.discourse-cdn.com/v4/letter/j/f05b48/32.png) [@JoelUpchurch](https://boards.straightdope.com/u/JoelUpchurch)\ **Post date:** [August 23, 2010, 3:09pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/25 "2010-08-23T15:09:33Z")
> [@Chronos](#): > > > [@](#): > > > > If you were to consider him the fourth, whom would you consider the third? > > Newton and Einstein are the top two, then maybe Archimedes, then Maxwell. Right people, but I think I would have to rank Archimedes ahead of Einstein. ---
**Author:** ![JoelUpchurch](https://avatars.discourse-cdn.com/v4/letter/j/f05b48/32.png) [@JoelUpchurch](https://boards.straightdope.com/u/JoelUpchurch)\ **Post date:** [August 23, 2010, 3:14pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/26 "2010-08-23T15:14:07Z")
> [@muldoonthief](#): > > > [@Chronos](#): > > > > Newton and Einstein are the top two, then maybe Archimedes, then Maxwell. > > How about Faraday? Experimentalists need love too, you know. Newton and Archimedes were great experimentalists also. Remember the prism? The reflecting telescope? Weighing the crown? The experiments with simple machines or reflected light? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 23, 2010, 3:18pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/27 "2010-08-23T15:18:57Z")
> [@](#): > > Right people, but I think I would have to rank Archimedes ahead of Einstein. I won’t debate that point: Fine distinctions can be unclear, when you get to that point. Faraday, though, while impressive, I don’t think has quite the same breadth of contributions as those other four. ---
**Author:** ![Raygun99](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@Raygun99](https://boards.straightdope.com/u/Raygun99)\ **Post date:** [August 23, 2010, 4:40pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/28 "2010-08-23T16:40:02Z")
> [@Jenaroph](#): > > > [@PSXer](#): > > > > my friend wants to know > > “Tootie.” “I just like saying that name.” ---
**Author:** ![Olentzero](https://avatars.discourse-cdn.com/v4/letter/o/90ced4/32.png) [@Olentzero](https://boards.straightdope.com/u/Olentzero)\ **Post date:** [August 23, 2010, 6:35pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/29 "2010-08-23T18:35:32Z")
We’ve kinda skirted around the question somewhat. Isn’t there a vacuum between the part that holds the beverage and the outer surface? That’s what I’d understood. ---
**Author:** ![qpw3141](https://avatars.discourse-cdn.com/v4/letter/q/6bbea6/32.png) [@qpw3141](https://boards.straightdope.com/u/qpw3141)\ **Post date:** [August 23, 2010, 6:41pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/30 "2010-08-23T18:41:12Z")
> [@Olentzero](#): > > We’ve kinda skirted around the question somewhat. Isn’t there a vacuum between the part that holds the beverage and the outer surface? That’s what I’d understood. The vacuum was between two silvered glass containers (the inner one holds the contents - the outer was just suspended in the overall container) in any that I’ve seen. Haven’t used one for many years, though. ---
**Author:** ![GHO57](https://avatars.discourse-cdn.com/v4/letter/g/df788c/32.png) [@GHO57](https://boards.straightdope.com/u/GHO57)\ **Post date:** [August 23, 2010, 6:43pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/31 "2010-08-23T18:43:13Z")
> [@Olentzero](#): > > We’ve kinda skirted around the question somewhat. Isn’t there a vacuum between the part that holds the beverage and the outer surface? That’s what I’d understood. Pretty much, a vacuum is an excellent thermal isolator; it stops convective and conductive heat loss. And he inside surfaces are usually mirrored to prevent radiative heat loss. ---
**Author:** ![kaylasdad99](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kaylasdad99/32/3398_2.png) [@kaylasdad99](https://boards.straightdope.com/u/kaylasdad99)\ **Post date:** [August 23, 2010, 6:56pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/32 "2010-08-23T18:56:39Z")
I once tried to take advantage of the fact that the thermos could do both, and put in some hot stew, and cold milk. It didn’t work very well. ---
**Author:** ![dzeiger](https://avatars.discourse-cdn.com/v4/letter/d/46a35a/32.png) [@dzeiger](https://boards.straightdope.com/u/dzeiger)\ **Post date:** [August 23, 2010, 7:00pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/33 "2010-08-23T19:00:08Z")
Found the Good Eats episode where Alton explains how thermoses work at [http://www.youtube.com/watch?v=sJC9Trw9iCQ](http://www.youtube.com/watch?v=sJC9Trw9iCQ) The thermos bit starts at about 7:30. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [August 23, 2010, 8:40pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/34 "2010-08-23T20:40:22Z")
> [@qpw3141](#): > > > [@Olentzero](#): > > > > We’ve kinda skirted around the question somewhat. Isn’t there a vacuum between the part that holds the beverage and the outer surface? That’s what I’d understood. > > The vacuum was between two silvered glass containers (the inner one holds the contents - the outer was just suspended in the overall container) in any that I’ve seen. Haven’t used one for many years, though. Glass thermoses are becoming rare for consumer products in my experience, too fragile. And good riddance to them. I remember the feeling when you could hear the sound of a glass cocoa slush as you pulled an old thermos dripping with rapidly cooling cocoa out of your backpack. Steel thermoses are the most common, at least in my neck of the woods. And it’s not really two containers, it’s one double walled container with a vacuum between the walls. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [August 23, 2010, 8:54pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/35 "2010-08-23T20:54:03Z")
Making a barrier to heat is easy. A barrier to cold - not so easy. Ultimately it’s because the speed of light is less than the speed of dark. ---
**Author:** ![Tibby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tibby/32/17030_2.png) [@Tibby](https://boards.straightdope.com/u/Tibby)\ **Post date:** [August 23, 2010, 10:21pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/36 "2010-08-23T22:21:28Z")
> [@panache45](#): > > > [@Shagnasty](#): > > > > My thermos has a switch on the side to choose between keeping things hot of cold. It works great and only cost $49.05 + shipping and handling from the shopping network. It never needs batteries either which is a huge bonus. I have no idea how it works though. Technology is an amazing thing. > > I have a battery-powered version. The amazing thing is that it still works when the batteries have died. ``` I purchased the remarkably thin walled Cold Fusion Activated Tepid Thermos from the esteemed Macrobiotic & Homeopathic Society for only $9.95 + $4.95 Shipping + $38,999.95 Handling. It included a 100% money back guarantee, including shipping—so you know they were on the up and up. I could not be happier—put in any beverage, hot or cold, and after 30 minutes, it turns healthily lukewarm. They even threw in a sample bag of brown rice and instructions on how to defecate while standing up. ``` ---
**Author:** ![GHO57](https://avatars.discourse-cdn.com/v4/letter/g/df788c/32.png) [@GHO57](https://boards.straightdope.com/u/GHO57)\ **Post date:** [August 23, 2010, 10:28pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/37 "2010-08-23T22:28:50Z")
> [@EdwardLost](#): > > Making a barrier to heat is easy. A barrier to cold - not so easy. Ultimately it’s because the speed of light is less than the speed of dark. about the speed of sound? ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [August 23, 2010, 10:46pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/38 "2010-08-23T22:46:09Z")
> [@EdwardLost](#): > > Making a barrier to heat is easy. A barrier to cold - not so easy. Ultimately it’s because the speed of light is less than the speed of dark. Makes sense, Wherever light goes, dark is always there first. ---
**Author:** ![Iggins](https://avatars.discourse-cdn.com/v4/letter/i/b9e5f3/32.png) [@Iggins](https://boards.straightdope.com/u/Iggins)\ **Post date:** [August 23, 2010, 11:19pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/39 "2010-08-23T23:19:56Z")
> [@GHO57](#): > > > [@EdwardLost](#): > > > > Making a barrier to heat is easy. A barrier to cold - not so easy. Ultimately it’s because the speed of light is less than the speed of dark. > > about the speed of sound? Close to the speed of smell. ---
**Author:** ![Superfluous\_Parentheses](https://avatars.discourse-cdn.com/v4/letter/s/8edcca/32.png) [@Superfluous\_Parentheses](https://boards.straightdope.com/u/Superfluous_Parentheses)\ **Post date:** [August 23, 2010, 11:27pm UTC](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114/40 "2010-08-23T23:27:14Z")
> [@EdwardLost](#): > > Making a barrier to heat is easy. A barrier to cold - not so easy. Ultimately it’s because the speed of light is less than the speed of dark. I think you mean [the speed of heavy](http://www.cartoonlair.com/i-am-weasel/s4e06-magnificient-motorbikini-video_2d30fc1f7.html). [Previous page](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114.md?page=1) [Next page](https://boards.straightdope.com/t/how-do-thermoses-know-if-they-are-supposed-to-keep-drinks-hot-or-cold/551114.md?page=3)