The AEG tethered helicopter is shown here :
http://www.aviastar.org/helicopters_eng/aeg.php
I may be wrong, but looking at the photo, at the shape of the rotor and at te guy to the left,
I have the feeling, that this photo was taken earlier, than during the 1930s.
But I was wrong, too, with my memories, as I actually found that article about a tethered helicopter
from WW I: It wasn't a German, but an Austrian-Hungarian design, drawn by Lieutenant Petrócz in
1916, it wasn't electrially powered, but by three rotary engines and built the Öffag (Österreichische
Flugzeugfabrik A.G.). And it had contra rotating rotors, see phot from Flugzeug Classic July 2008.
But the system was already tried in WW I !