# Resistance wire question (nichrome)



## stevehammy (Mar 7, 2013)

I'm designing a device which will be used to test the reaction of various materials when a red hot heat source is brought to within about 1/16" of the surface of the material. 

I'm using a 1" length of 29 gauge nichrome wire bent into a very tight "V" shape... the pointy end of which needs to be glowing red hot, but NOT the upper half of the nichrome legs. It is connected to a power source which allows me to adjust the voltage, thereby adjusting the temperature. 

The problem is that just the pointy end should be red hot, and the rest of the nichrome needs to be cooler. I have seen similar devices where only the point is red hot, but in my case the entire length of nichrome glows fairly evenly. Turning down the voltage does not help, it just lowers the overall temperature evenly.


+ - nichrome resistance wire connected to variable DC
| | 
| | <-- this part should be cooler
| |
| | <-- this part should glow red-hot
V
________ surface to be tested for reaction to heat


So this is my question:
Is there a technique to adjust the resistance (or somehow treat) the nichrome wire so only the point glows red and the upper 1/2" remains cooler? I know it can be done because I have a sample of a component apparently made of resistive wire which does just that. My part has the same physical shape and appearance, but mine glows evenly. Perhaps another material like stainless steel would give the desired results? Any ideas?
Thanks,
Steve


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## sandman55 (May 28, 2005)

The only way you would control the heating of the length of nichrome wire is to increase the diameter of the nichrome wire on the section that you want to remain cooler. This can be done by doubling up or even tripling up the section to remain cooler and then fuse them together with silver solder leaving the middle part thinner and hotter.


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