Spaulding & Rogers powerunit 1

Hi all! Perhaps you can help me out with my Spaulding & Rogers Power Unit 1. Bought it via eBay and it was in good shape, got a powerconverter to covert our 230v to 110v with a max of 200 W. Maybe it is how the unit works, but if I turn the rheostat down to 1 and hook on a machine, it still spits out so much power, there is a blue spark between the spring and the contact screw. A little contactspray fixes that, but when I measure the power with a digital multimeter it reads 8,3 V even with the rheostat down to 1. I am beginning to think the power unit just works this way, and that the rheostat is for extra voltage. However, I had to rebuild all machines to be able to use it with the power unit and I found out I can give much more tension to the springs and the machines run stable without any change. But it is kinda confusing. Could the rheostat be broken because it gives out 8,3 volts all the time, or does the power unit from Spaulding just work this way ? The guy from eBay mentioned in his add it was not so easy to tattoo with this unit, might that be the reason? Any help would be greatly apreciated! Gilbert