An arc of flame shows you the path of an electric current. When you press the button, one side of each pair of electrodes becomes positively charged, and the other side becomes negatively charged. Since opposite charges attract one another, when you get the electrodes close enough, the charges will jump across the gap. The glowing light—the arc-shaped flame that you see—marks the path that the current takes. The flame is arc-shaped because of the heat it creates. Warm air rises: you can feel it when you put your hand above the chamber. The drafts of hot air made by the heat of the arc not only pull the flame upward in the middle, they also make it possible for the flame to jump up from the bottom pair of electrodes to the top pair.