An old Eastern German generator, standard issue for most Czechoslovakian bikes. It is somewhat weaker than today's generators, but still just reaches its nominal values of 6 V / 3 W.
One pole connects via a nut, the other one goes through the frame. The driving cog can be removed after unscrewing a locknut. You'll find one end of the plain bearing under it, where you can drip some oil. That's all maintenance that can be done - no further disassembly is possible.
The output is independent on wheel size, the tyre always travels at the speed of the road. The current was rectified by a diode bridge and smoothed out by a capacitor for these measurements.
Open circuit voltage reaches 41 V at 35 km/h and if I could pedal faster, it would keep growing.
Maximum short circuit current is 480 mA and doesn't get any higher.
Exactly 3 W at 20 km/h and 40..50 Ω of load.
I was quite surprised by this veteran's performance. I withdraw my previous remarks about inevitable fading of old magnets, it's not that bad after all. Compared to contemporary production, this dynamo probably has greater internal resistance: at 35 km/h it generates 100 mA less short circuit current and some half watt less with an optimal load. But it is only marginally weaker in the usual working range up to 20 km/h.
I don't know how the Ruhla behaves in practical use, wet weather etc. - I came, I measured, I uninstalled. The cog looks like ceramic rather than plastic, so it should wear slowly. The combination of no-maintenance internals and no-seal plain bearing scares me a bit. There was quite a lot of rolling resistance during the test rides, but that was caused by me having forgotten to lubricate the shaft first. Final conclusion: no first class, but still usable.
Source data to download (XLS, Excel 97). This time I interconnected the tables better, so all you need to do is to fill your values in one place and all the figures will be plotted automatically.