The the choke fully inserted the current draw from the mains (240v) is limited to about 20A,
with the core fully extracted the current is limited to around 70A (Without the Ballast connected the full load current is 85A).

By setting how far out the potted core sits, the maximum current can be set anywhere from 20-70A. The ballast is made of about 300-400 2mm welding rods with the flux stripped off and potted using a clear casting resin.

Core of Sliding Choke Ballast, Made from Hundreds of 2mm Welding Rods, encased in casting resin. The worst part about making this was stripping the flux off 20 odd kg of welding rods. (Which were picked up for next to nothing at a welding supplied shop, I just asked for any water damaged ones etc).  I found the best way to strip the flux was to use a cordless drill at the bare end of each rod and just run pliers over the rod as at was spinning.  After there was enough stripped rods to mostly fill the PVC pipe (with them taped tightly together) They are then potted in an off-cut 65mm piece. I used silicon spray to ensure that the mould would be easy to remove.

Coil Form for Core to Slide into. To allow inductance to be varied. Consists of approximately 160 turns of 4mm^2 wire. Wound on a 65mm PVC Pipe. It is wound in 4 layers to get the number of turns. The winding is the same length as the Sliding core.

The current limiting is set by inserting a piece of wood the correct length into the PVC pipe before the core itself hence raising it to the desired level. The higher the core sits the more current that flows.