Model tests to investigate the effects of geometrical imperfections on the NIST calculable capacitor

The calculable capacitor at National Institute of Standards and Technology (MST) links the U.S. capacitance unit to the SI unit and has a relative standard uncertainty of 2 x 10(-8) Geometrical imperfections are one of the largest sources of this uncertainty. Tests with a model calculable capacitor have been done to better evaluate and reduce this uncertainty. These included evaluations of the Effect of eccentricity of the blocking electrodes used to define the capacitor's length and of uniform taper on all the calculable capacitor bars. In addition, the effect of tilt of the blocking electrode and taper in three or fewer bars was investigated. A new cone-shape for the tip of the blocking electrode was also studied.