Two types of shielded variable air condensers have been investigated in order to determine which would give: (1) the smallest absolute minimum capacity with a variation of about 20 μμ F.; (2) the largest ratio of maximum to minimum capacity, with a variation of about 60 μμ F.
The best values actually obtained for the minima in these two cases were 3.5 μμ F. and 5.3 μμ F. respectively.
It was found that flat semicircular plates were superior to cylindrical segmental ones. The lowest minimum was obtained when the moving system was insulated on the underside of the top bearing.
Disconnexion and earth-capacity errors have been fully determined, and these indicate that the absolute values of capacity can be relied upon to less than 0.2 μμ F.