Step 12: Turning it on
Turn the dimmer all the way down (counterclockwise).
With your fire extinguisher handy and your body away from the device, plug it in. Are the lights still on? Is the hotwire still whole? Sweet.
You can use your multimeter to see if there's any current between the terminals. There shouldn't be yet. SLOWLY (like 5 degrees per second) turn the dimmer up (clockwise) til the wire starts to quietly hum. With the wire I used that's about 1/4 or 1/3 of the full rotation. If the wire doesn't hum or heat up by the time the dimmer is halfway up, turn the dimmer all the way down, push in til it clicks, and start again.
If you turn up the dimmer too fast, your wire may burn out before you realize that it's even hot.
Once you are sure everything's all set, grab the frame and try laying the wire on some scrap styrofoam. It should slice smoothly into the foam. You shouldn't have to push very hard. Try playing with the dimmer switch setting to get the best cut. I've read that cutting slower and cooler makes a smoother cut.
The wire heats up and cools down within a second or two.
Next time you use the foam cutter, make sure nothing meltable or flammable is touching the wire when you plug it in, just in case the dimmer isn't off.