Help: how to design temperature control for heating mantle

[Prune]

I want to make a heating mantle (for lab use) as I'm too poor to buy one (after buying glassware, no $ left...). I got some 18 AWG nichrome wire and Zetex fiberglass loom tubing from eBay for the heater. Now, first of all I'm not sure how to calculate the length of nichrome to use as a coil, given a certain wattage I want, and some minimum length (so I can wind a large enough mantle). A complication is that the resistance of the nichrome is a function of its temperature. My goal is maximum output of at least 300 W (500 W is better) and a coil that will fit in one to three feet of the fiberglass (no problem in having, say, a couple of loops in parallel).

The other thing I need help with is how to adjust the output. Some use variacs, but my goal here is to save money. Finally, I'd like to use a thermistor based temperature probe to control the thing. I plan to run it directly off the mains (so I don't have to get a transformer).

So, how do I do this? I could use a dimmer from a lamp for control, but then I can't automate it with a temperature probe. Please guys, help me out here.

BTW, will nichrome interfere with a magnetic field? I plan to DIY a magnetic stirrer that will sit under the mantle. It may be a problem if it keeps moving the coils.

[leadbelly]

I believe that the simplest circuit approach would be:

thermistor -> op amp -> potentiometer -> pulse width modulator -> opto-coupler -> triac

the op amp is to condition the thermistor input
the potentiometer is to act as a temperature setpoint by being a voltage divider so that you adjust the 0 to 1 transition of the trigger to the PWM
you need the PWM to cycle the coil
you need the opto-coupler to connect low voltage digital to AC
you need the triac to switch the "big" AC load