  The most common method to measure direct current high voltage (HV) down to
the ppm-level is to use resistive high-voltage dividers. Such devices scale the
HV into a range where it can be compared with precision digital voltmeters to
reference voltages sources, which can be traced back to Josephson voltage
standards. So far the calibration of the scale factors of HV dividers for
voltages above 1~kV could only be done at metrology institutes and sometimes
involves round-robin tests among several institutions to get reliable results.
Here we present a novel absolute calibration method based on the measurement of
a differential scale factor, which can be performed with commercial equipment
and outside metrology institutes. We demonstrate that reproducible measurements
up to 35~kV can be performed with relative uncertainties below $1\cdot10^{-6}$.
This method is not restricted to metrology institutes and offers the
possibility to determine the linearity of high-voltage dividers for a wide
range of applications.
