DC polarity wiring for Dell connector

[malco]

Hi
I have to build a DC-DC converter to power a Dell D800. The Dell leads I have been given (7m/m diameter 3 wire plug) have black and white wires and signal Which is the correct polarity? is the outer shell negative?

 

[frozenlead]

If you would have posted this just a few hours ago I could have pulled one out and told you, but I moved back to school...