now i found the cause, i think: :
when i measure the voltage with my arduino-oscilloscope
(it can do a sample rate of 7kHz to 8kHz at 500kBaud)
at the end of the 10m-line,
i get a cloud of values (no 50Hz wave, but much higher frequency)...
that cloud averages to the right value...
but when i add a capacitor
(i tried a red box-shaped plastic-foil-capacitor with 6.8uF, too),
i get 20mV less...
why is that?
i thought, capacitors would be nice "averagers"... :
should i better ask in an electronics forum,
since this is surely not arduino related
(the arduino measures fine)?