This is a Wechselstrommaschine, an alternator. The wheel goes around, and the spokes hit contacts – one set makes the electricity go in one direction and the other goes in the opposite direction. It alternates. That’s how AC power – alternating current power – works.
The image comes from Bibliothek allgemeinen und praktischen Wissens für Militäranwärter Band III, an encyclopedia of practical, general knowledge for military trainees. I guess you should know how to improvise an alternator on a pre-Great War battlefield?
I found it on Wikimedia Commons.