Determination of the SI Volt at the PTB

Work on an electrostatic measuring system for the determination of the SI volt and the conversion factor KV at the PTB is reported. The system consists of a so-called voltage balance and a generator for the dc measuring voltage. There is a need for the development of such a device with an uncertainty below 1 part in 106 because measurements with existing voltage balances, and results obtained by other methods show discrepancies of several parts in 106. In order to reduce the uncertainty and to simplify the apparatus a special measuring method was developed. The voltage/force transducer of the PTB voltage balance is formed from two coaxial cylindrical electrodes and the generated force is substituted for the force of gravity on a weight of 2 g. The electrode voltage is composed of a constant part of 10186 V, derived in a 1000-fold stepup from 10 standard cells, and a much smaller variable part used for balancing the scale beam by means of a control loop. Taking the root sum of squares the total relative uncertainty of the SI volt and of KV is expected to be less than 4 parts in 107.