Definition of "arcing"

Whenever the locomotive was working hard there was unmistakable evidence of its higher power than its predecessors in the brilliant and explosive arcing between conductor shoes and the third rail; this was particularly vivid in Quarry Tunnel in the down direction, where the display equalled anything we have seen on the frostiest of nights in an electrified third-rail area.

1959 March, “The 2,500 h.p. electric locomotives for the Kent Coast electrification”, in Trains Illustrated, page 125