The surfaces of the spheres in the neighbourhood of the sparking points
shall be free from any trace of varnish, grease or other protective coating.
They shall be clean and dry, but need not to be polished. If the spheres become excessively roughened or pitted in use, they shall be refinished or replaced.For relative air humidity exceeding 90 per cent, moisture may condense on
the surface and the measurement will then cease to be accurate