I picked up a little sheet metal roll with 30" rolls recently. It is old
and had been neglected. There was rust on the rolls. I disassembled the
machine and decided to do electrolytic derusting on the worst of the rolls,
followed by polishing in the lathe.

The first problem was what vessel to use. A workpiece 32" long and slightly
less than 2" is quite awkward. I wound up cobbling. I used two 3' lengths of
scrap 4x4 wood tacked together on the ends with scrap 1x4 wood to make a
frame of sides only. Then I took a garbage bag and put it down inside the
frame so it hung over the edges, then stapled it down. I placed sheet lead
down the length of my "gutter" with tabs that stuck up for connection points,
then put little wood spacers at each end and put the workpiece in. I filled
it with solution from my regular bucket, clipped on the leads from the battery
charger, and fired it up. It bubbled nicely but the ammeter was clipped on high
even on the 6 volt setting. In a few seconds, the overload in the charger
tripped. I took the workpiece out and bailed out most of the liquid and replaced
it with plain water, hoping to dilute the solution.

That got it down to about 16 amps, but the little charger is only a 10 amp
unit so after several minutes it popped again.

That's when I got my little idea!

I put a 20A variac in front of the battery charger and turned it down so the
charger only saw about 60% of its input voltage. Ah hah - can set the current
right on 10 amps! There were two drawbacks with using a $5 garage sale car
battery charger. One is that it won't deliver a whole bunch of current to do
e.g. a truck bed. The other is you only have one switch to control the current -
the 6V/12V toggle. Sure, you can monkey with your solution, or your topology,
but in this case monkeying with the topology would have meant starting over.

This is another handy use for the variac.

Grant Erwin
Kirkland, Washington