Calculable capacitor
In collaboration with the NMIA of Australia, the BIPM is building two calculable capacitors of an improved design to measure the von Klitzing constant with an uncertainty of, the order of, 1 part in 108 to contribute to the next CODATA fundamental constants adjustment.
In September 2007 we received the two frequency doubled Nd:YVO4 lasers needed to measure, by interferometry, the electrode spacing in the two instruments. The characteristics of the laser beams have been studied with a wave front sensor to determine the optical system needed to couple the laser beam into a fibre delivering the light to the interferometer. A test bench for studying the properties of the interferometer has been set up and first measurements of the interference fringes were made. The electrical measurement systems, which will link the quantized Hall resistance with the calculable capacitance, have been improved to reduce the related uncertainty, which is expected to dominate the uncertainty of this experiment. The workshop of the BIPM has continued to fabricate a large number of parts.
Watt balance
During the period of this report, the staffing situation has improved with the arrival of an assistant, who works nearly full-time on the project, a research fellow on a two year contract and a secondment from the NMIJ who worked at the BIPM for seven months. We have continued the development of the room temperature experiment, which will be followed at a later stage by a cryogenic experiment, to test the feasibility of simultaneous force and velocity measurements.
The main objective of the year 2007 was to carry out the first measurements of the voltage-velocity ratio, which has been achieved with a relative standard deviation between different measurements of, the order of, 1 part in 104. The coil suspension has been improved to reduce the undesired coil movement in the five degrees of freedom resulting from a non perfect vertical movement. Further work on the current source has allowed us to reduce its long-term drift to about 1 part in 109 per minute, and to achieve a satisfactory short-term stability. A technique needed to separate the voltage induced in the coil from the voltage drop due to the current flow, based on the used of a second, non-inductive coil, has been tested and is being integrating into the apparatus.
Work has started on the geometrical and magnetic characterization of a large precision solenoid which will become the reference for the magnetic field alignment.
The collaboration with the Machine Tools Department of the Technical University of Aachen (RWTH/WZL) on the fabrication of the magnet is continuing. During the year 2008, detailed mechanical drawings for the fabrication of the magnetic circuit and the assembly devices will be provided. Preparations have been made for the installation of a vibration isolation base in the future watt balance laboratory.
Summary
