  The KATRIN (KArlsruhe TRItium Neutrino) experiment aims to determine the mass
of the electron antineutrino with a sensitivity of 200meV by precisely
measuring the electron spectrum of the tritium beta decay. This will be done by
the use of a retarding spectrometer of the MAC-E-Filter type. To achieve the
desired sensitivity the stability of the retarding potential of -18.6kV has to
be monitored with a precision of 3ppm over at least two months. Since this is
not feasible with commercial devices, two ppm-class high voltage dividers were
developed, following the concept of the standard divider for DC voltages of up
to 100kV of the Physikalisch-Technische Bundesanstalt (PTB). In order to reach
such high accuracies different effects have to be considered. The two most
important ones are the temperature dependence of resistance and leakage
currents, caused by insulators or corona discharges. For the second divider
improvements were made concerning the high-precision resistors and the thermal
design of the divider. The improved resistors are the result of a cooperation
with the manufacturer. The design improvements, the investigation and the
selection of the resistors, the built-in ripple probe and the calibrations at
PTB will be reported here. The latter demonstrated a stability of about
0.1ppm/month over a period of two years.
