The spheres shall be carefully made so that their surfaces are smooth and their
curvature is as uniform as possible. The diameter shall nowhere differ by more
than 2 per cent from the nominal value. They should be reasonably free from
surface irregularities in the region of the sparking points. This region is defined
by a circle such as would be drawn on the spheres by a pair of dividers set
to an opening of 0.3D and centred on the sparking point. The freedom from
surface irregularities shall be checked by adequate measuring devices (for
more details see reference 5 or 6).