It's true that metals will increase in resistance as they are heated (just the opposite is true of semiconductors), however, the alpha (temperature coefficient) of NiChrome (the same wire used to heat the cutting stylus of a disc mastering lathe, btw) is extremely small. It is about 0.00017. Therefore, even if the 1700 degree temperature were meant to be in Centigrade, which I gather it was not, the current measured should be no lower than 19 Amperes - more likely 21... This is because the 0.8 Ohms (it was 8/10 Ohm, wasn't it? You might have mistyped it in the second post, which reads simply "8 Ohms") per foot, multiplied by 6 gets heated up from a 20-degree Centigrade reference temperature to resist current at a level of around 6 Ohms - whence the 21 Ampere charge flow. If you ask me, 25 is closer to 21 than 12 is. 4 Amps versus 9. Ergo, again, something is not right in the figures we have been given.
Let's pretend, anyway, that the current drawn will actually be 21 Amperes at 1700 degrees Fahrenheit (i.e., 927 degrees Centigrade). At 375 degrees Fahrenheit (190.5 degrees Centigrade), the original 6 feet of NiChrome wire has risen in resistance from 4.8 Ohms to 4.9 Ohms. At 12 feet of NiChrome wire, the resistance at 375 degrees Fahrenheit should be about 9.8 Ohms. If the Power to dissipate 1700 degrees from NiChrome was 120 Volts squared divided by 6 Ohms = 2400 Watts, then we might be able to divide this figure by the quotient of 1700/375 (= 4.5) and apply that in the calculation of the required current for the lower temperature. Then we can know how much potential energy to apply across the NiChrome wire.
2400 / 4.5 = 533 Watts. The current that should get us to the temperature dissipation we are looking for should be around (533/120 =) 4.5 Amperes. So, the potential energy you need to apply to get this to work with 12 feet of NiChrome wire should be 44.1 V.
I hope this helps you.
- Mesmer8