Calculable capacitor
The BIPM workshop continues to fabricate components for the two calculable capacitors being developed in collaboration with the NMIA. Two members of the Electricity section have spent three weeks at the NMIA to learn about the project and to optimize the shape of the guard electrodes. A member of the Time, Frequency and Gravimetry section has visited the NMIA to discuss questions related to the interferometer needed to measure the relative position of the guard electrodes. Agreement has been reached on the general approach. The mode coupling optics will be developed at the BIPM during this year and we have started to set up an interferometric test bench to study the contribution of the interferometer to the total uncertainty of the system. The chain linking the quantum Hall resistance to capacitance standards has been improved. A first electrode bar has been fabricated at the NMIA with a cylindrical geometry within the specifications. It is planned that our instrument shall arrive at the BIPM in 2008.
Watt balance
A cooperative venture has been started with the Technical University of Aachen (Germany), with the objective of fabricating and assembling the magnetic circuit for the watt balance. This is particularly difficult because very small mechanical tolerances of the pole pieces are needed. The balance suspension has been re-designed for greater stiffness and to correct some other mechanical imperfections. An electrostatic damping system was added to stabilize the coil against rotations around the vertical axis, which should reduce the velocity noise during the vertical movement of the coil. The interferometer is being integrated into the system so that, in future, velocity readings can be obtained directly from the interferometer. An independent technique to verify the velocity readings of the interferometer, based on the measurement of the Doppler shift, has been set up. A first version of the moving coil has been built and inserted into the test magnet. We have developed a technique for precisely synchronising the measurements of the induced voltage by the digital voltmeter and that of the velocity by the interferometer, so that noise components common in both signals are suppressed in the voltage-velocity ratio. The elasto-mechanical properties of the ground have been determined for the conception of an anti-vibration base.
Summary
