The accurate measurement of voltages in the range of ten to several hundred kilovolts is not only of interest for national standards organizations like the Physikalisch-Technische Bundesanstalt, but also for high precision experiments like KATRIN that is supposed to measure an upper limit of the neutrino mass by measuring the energy spectrum of the tritium beta decay. Therefore, a voltage of 18 kV has to be measured with maximum accuracy. Another application is the laser spectroscopy at storage rings, like ESR at GSI Darmstadt. To measure the unknown transition frequencies of ions correctly, the voltage of the electron cooler has to be calibrated, because this voltage mainly determines the velocity of the ions. This is important for the determination of rest-frame frequencies of unknown atomic transitions to test fundamental theories. (to main page >>)