Since you've obviously already purchased this 50 kV,
400 mA unit, my suggestion would be to go ahead and
plan on using it for somehting like a Jacob's ladder, once
you've given it a nice tank of oil to run in. It was most
likley originally in an oil-filled tank and was removed
for ease of storage, shipment, ect, but you will almost
certainly need to "repot" it if you intend to run it at its
nameplate voltage rating to prevent destructive flashover
of the high voltage windings. I use a nice sized x-ray
transformer to drive a wicked Jacob's ladder here:
http://www.youtube.com/watch?v=0dLTEOM3qQc