Final answer:
When an arc is struck, the high temperature and energy of the arc ionize the surrounding air and materials, creating an electrically conductive plasma that allows current to flow.
Step-by-step explanation:
When an electrode coating is dry, it acts as an insulator and does not conduct electricity. However, when an arc is struck, the high temperature and energy of the arc can cause the air and materials nearby to become ionized. This ionization process creates electrically conductive plasma, allowing current to flow. In air, this breakdown of insulating properties and transition to a conductive state occurs at around 3×106 V/m. Similarly, materials like selenium exhibit photoconductive properties, becoming conductive when exposed to light, as opposed to their insulating state in darkness.