A Bias Source for the Voltage Reference of the BIPM Watt Balance

We have developed a programmable current bias source to set the voltage output of a Josephson voltage standard based on a nonhysteretic array of Josephson junctions (JJs). The device will be operated on the International Bureau of Weights and Measures (BIPM) watt balance experiment and has been designed to bias separately each of the 13 independent subarrays of the superconductor-normal-metal-superconductor JJs to be able to track the induced voltage across the moving coil during the acceleration phases. At the time that the constant velocity mode is reached, the voltage difference measurement setup will be ready for data acquisition. The current bias source is powered by lithium-ion batteries to avoid electrical offsets originating from ground loops and to allow a long series of measurements. We report the results of the first tests carried out since the assembly of the current source.