Ball bearings have always fascinated me, since manufacturing them still has an element of artistry about it. Roll a large number of them about long enough between parallel grinding plates and they come out statistically spherical and smooth within controllable tolerances.
If however you want to know how far from perfect a given ball is (perhaps for a very high performance application), today’s invention may help.
When a perfect conducting sphere is subject to a very high voltage it will, theoretically, distribute charge uniformly across its surface. Regions with locally sharper radius will concentrate charge.
Imagine a machine, therefore, in which each candidate sphere is charged up on a non-conducting plate in a dry-air enclosure, using a very high-voltage source.
Approach the sphere with a micrometer-driven conducting wand. Record the proximity at which discharge by air breakdown occurs. Sharper regions will discharge at greater distances from the wand.
This can be repeated for a large number of different rotational positions to ensure that each ball is spherical to within ultra-fine limits.
(The wand might contain a tiny reservoir of ink, so that the sharpest point on each ball might be highlighted by attracting a dot of oppositely-charged pigment).