+1, if you look at the Schmidt hub, in the linked picture.. there is a black ring separating the 2 parts of the axle end.

that's an insulator .. the + terminal is the flange , meets the hot lead face of the dropout inlay..


center of the axle is the ground.


NB:
Just a heads up .. I had my disc brake dynamo hub creep around in rotation , wrapping the cable around the axle .

when the QR skewer was a bit loose..

with the wire short, and fixed in a thru the fork routing , the plug may turn forward

further than the wire slack allows, and you will break the circuit. and need to rewire your fork ..

I had enough extra slack and my tape and zip tie job allowed the wrap up to pull some extra, without damage.


(I'm thinking the magnetic induction drag added up, over the miles,
rather than the disc brake part of the wheel doing the windup.)