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How to get 300 degrees? Answered

 The Makezine Weekend Project on the portable heater said that 3 volts (two AA batteries) with 30 feet of 30 gauge wire will give 100 degrees F. Will the same setup with only 10 feet give 300 degrees? If not, is there a way to do this with minimal changes? Either way, will the setup do anything annoying like the wire stop heating and the batteries start heating up or the whole thing melting? Thanks!


Do you want to make a heater?
AA batteries might get something to a temperature, but they're a bit weedy for heat if you're cold.


 sort of. i'm trying to make an electric marshmallow cooker and i determined that 300 degrees is enough to cook them. i had given up before but when i saw the weekend project i got new hope.

I fit helps, I just hooked up a voltmeter to either side of the batteries (which gave about three volts) and then wired the circuit. The voltage immediately dropped to about 2 degrees and slowly declined until I unwired the circuit and the voltage increased to a little less than 3 volts.

That's a good idea. If you've got good batteries that can deliver plenty of current, something like an old toaster-element maybe what you need.


The math is not as simple as you suggest (temperature in F being inversely proportional to the length of wire), but you do have the right idea.  Decreasing the length of wire, by 1/3,  will decrease its total resistance, also by 1/3. 

Assuming this resistance is much larger than the internal resistance of the battery, the power dissipated by the wire, V2/R, is going to increase by 3. 

Also the total surface area of the wire over which it can dissipate power has decreased by 1/3.

Anyway, the math isn't easy, but I claim that it is possible to achieve high temperatures from AA batteries, although with limited total power.  As proof of this claim, I offer a link to a description of a surgical cauterizing tool that runs on AA batteries.  The tip of this tool reaches 2200 F, orange hot.


Regarding the question of  "the wire stop heating and the batteries start heating up" , this condition occurs when the resistance of the load, RL, your piece of wire, is smaller than the internal resistance  of the batteries,Rs .  That is to say there is a limit to how small you can make the resistance of the wire. 

That's why I said "assuming this resistance is much larger than the internal resistance of the battery"

See also:


I love it when I learn something on this site. I would have assumed that decreasing the resistance by shortening the the wire would lower the temperature. Of course, I also believe that there are tiny people inside my radio, so what do I know?

The tiny people aren't in the radio.  They're in myyour head -- they have walkie-talkies which pick up signals from the radio and tell them what to say. 

Right now they're saying, "It's the end of the world as we know it, so kill the fire head," but I'm not listening to them...

You could ether increase the voltege or use thinner wire but useing less wire I dont think will increase the heat.