A spark will jump between plus and minus electrodes spaced at a certain distance when the voltage applied to them is increased. If you look closely, you will notice the section with a high electric field will emit a faint light (corona discharge) before the spark jumps. Saint Elmo's Fire is corona discharge that occurs when thunderclouds cause a sudden increase in electric potential differences in the atmosphere. Corona discharge occurs at an electric potential difference of about 100 V per centimeter. At about 1,000 V, Saint Elmo's Fire and other phenomena occur, and at 10,000 V or more, spark discharge, including lightning, occurs. Corona discharge occurs particularly easily at the tip of long objects pointed upwards into the atmosphere when a point discharge current is flowing into them.