The first time i saw a gherkin glow was a moment i shall long remember.
Tim Hunkin stood with cables raised aloft like a latter day Frankenstein and produced an eeire glow and stunned silence.
Later i was to electrocute many a wally myself. Including this one.
Where does the glow come from?
The gherkins are pickled and the salt in the pickling juice is responsible for the glow. Electrons in the sodium atoms get excited and when they fall back to their original levels, photons are released. These give a characteristic yellow/orange colour.
The phenomenon was described in the wonderfully named paper
Sodium D Line Emissions from Pickles” by J.R. Appling, F.J. Yonke, R.A. Edgington, and S. Jacobs, in Journal of Chemical Education, volume 70, number 3 (March, 1993), pages 250-251.