This design will look familiar after we think back to our science fair days when we dimmed a light bulb with a coil of wire and a steel rod. It worked then, and since it ain't broke, I won't fix it, and I'll draft it for service here at AAA Welder. The operation is simple. When the control rod is fully retracted, the only effect on the circuit is that of an air core inductor. When the control rod is fully inserted, the coils' "reactance" is maximized.
My goal in building this reactor was to exact control over a small range of power, 10 to 15 amps to fine tune my welder and to stabilize the arc. If too many turns are put on initially, the high end amperage will be reduced significantly before the rod is even inserted. The method I used was to wind the coil with the control rod fully inserted until the amperage dropped just below the target amperage. From there, it was a simple matter to retract the control rod until I had the exact amperage I wanted. I then locked the rod in place with the ring clamp.
Gathering the parts for this build was easy. Except for a solid steel rod, I found everything I needed in my garage.
In leiu of a steel rod, I cut 5 inches of 13/16" O.D. tube steel from an old broom handle, filled that with Iron powder and sealed the tube with a wooden plug on one end and a hot glue plug in the other. The coil was formed on a 3 inch piece of 1" PVC pipe that measured 15/16" I.D.. The windings were stabilized with hot glue, as they were wrapped, to keep them in place.
The final step for finishing the PVC coil former is to take a hacksaw and cut 4 slots on the Cardinal points, N-S-E-W, so that when the ring clamp is installed on the PVC, the PVC will compress onto the steel tube and lock it in place.
This design may not posess futuristic complexity but, it works perfectly. With no moving parts to wear out, it should serve faithfully and indefinately.