Details on the progress during the last year can be found in the most recent BIPM Director's report. Some additional experimental details and photographs are shown below. A presentation was made at the CPEM conference in 2006 (A. Picard et al., IEEE Trans. Instrum. Meas., 2007, 56(2), 538-541).

Balance suspension and electrostatic motor

We have built the suspension carrying the coil and the mass. It includes close to its upper end the electrostatic motor used to move the coil through the magnet. This device consists of a fixed electrode connected to ground, and two mobile electrodes on either side. Two different high voltages can be applied to the mobile electrodes. The electrostatic forces between both of the high voltage electrodes and the grounded electrode at the centre allow to move the mobile electrodes up and down. Three lever arms fixed with flexure strips transmit the movement of the electrodes to the coil hanging below.

Optical systems to measure the coil displacements

Optical systems based on position sensitive detectors (PSDs) are used to measure the coil displacements for all degrees of freedom (three translations and rotations around three axis). The vertical coil movement and the related velocity will also be measured with an interferometer.

Electrostatic dampers have been installed to avoid rotation of the coil around the vertical axes.

The magnet

The design of our magnet circuit includes two disks of Sm₂Co₁₇ magnets, magnetized in opposite directions, as the flux source. The yoke will be made of an FeNi alloy, which remains mechanically stable at cryogenic temperature. The circular air gap will have a diameter of 250 mm and a width of 13 mm. The magnetic field will have radial symmetric and the flux density will be close to 0.5 T.

One of the properties of the system is that the magnets and the air gap are completely screened by the high permeability iron yoke. This should reduce considerably the level of electromagnetic perturbations detected by the coil. The symmetry of the circuit with respect to the horizontal plane help in improving the uniformity of the flux density in the air gap.

We are currently working on the details of the mechanical design and the machining techniques required to realize the necessary very small tolerances. At present a simpler magnet is installed in the experiment as shown in the photographs above.