  A capacitance standard based on the definition of capacitance C = Ne/U is
realized when a capacitor is charged with a known number N of electrons (e is
the elementary charge) and the voltage U across the capacitor is measured. If U
is measured by means of a Josephson voltage standard, C is solely determined by
quantum numbers and fundamental constants. Presently, PTB is setting up this
experiment using a special type of single-electron pump, the so-called r-pump,
for transferring electrons one by one onto a cryogenic vacuum capacitor with
the decadic capacitance C = 1 pF. We report on the progress in the setup with
emphasis on the characterization of the single-electron tunneling elements. We
describe the modifications of the single-electron circuit that are necessary to
implement the capacitor charging experiment by using a four- or five-junction
r-pump.
