When a voltage across an insulating gas is increased beyond a certain limit, the gas breaks down, becoming a conductor of electricity. A photographic method, the Lichtenberg figure technique, is used to study the process. Above a certain voltage an ionization wave, called a streamer, proceeds from the highly stressed electrode, branching out along the way and extending far into low-field regions. At elevated voltages the vigorous streamers reach the cathode with a high-potential front. Owing to the front, the short-lived local field at the cathode triggers electron emission, also producing negative streamers. The negative streamers greatly increase the density of the ionized particles in the channel, yielding what is known as a backstroke. For long air gaps, streamers are incapable of reaching the cathode; intensive secondary channels therefore develop at the anode and proceed toward the cathode. Either the backstroke or the secondary channel paves the way for the full ionization of the channel and spark breakdown.