PeLady, iffen for a bicycle, the generator is also attached to the bicycle frame, ..... thus the frame is the ground.
Many vehicle and/or tractor lights are single wire ...... with the frame being the "ground".
Check your automobile, the negative (-) terminal of the battery is usually bolted directly to the metal frame. 
It did occur to me, but I wasn't certain about old time stuff. +1 Cogar.
