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The geometry and construction of electromechanical systems based on the levitation of a superconducting mass strongly influence the level of uncertainty they can achieve in realizing the "mechanical" joule. The authors give some recommendations for designers of systems of the next generation based on their experience with the apparatus at the VNIIM.

To monitor the stability of the kilogram a total uncertainty not exceeding 1 to 2 parts in 108 is required. Although a comparison of "mechanical" and "electrical" energy by the superconducting magnetic levitation method is viable in principle for this purpose, a more elaborate design of electromechanical system for levitation is required. A design for the system based on a combination of flat elements is suggested. The single element takes the form of a flat rectangular coil, produced by photolithography, and a superconducting plane located near and parallel to the coil. The characteristics of the element have been calculated in two versions. The first is meant for use in levitation, the second as part of a magnetic guide.

Directly comparing the "mechanical" with the "electrical" joule by the levitated mass method implies accurately measuring the vertical displacement of the centre of gravity of a levitated superconducting body. The required accuracy of some parts in 109 over relatively small displacements of a few millimetres when the stability of a standard mass is to be monitored, and the special conditions under which the measurements must be made, pose a number of specific problems. The principles of the D. I. Mendeleyev Institute for Metrology measurements system are discussed. Estimates of the potential of the system, based upon some preliminary results, are presented.