when a voltage is applied across the electrodes, the gas conducts almost no electric current until a threshold voltage is reached "(point b)", called the "firing" or "breakdown voltage", "v". at this voltage electrons in the gas are accelerated to a high enough speed to knock other electrons off gas atoms, which go on to knock off more electrons in a chain reaction. the gas in the bulb ionizes, starting a glow discharge, and its resistance drops to a low value. in its conducting state the current through the bulb is limited only by the external circuit. the voltage across the bulb drops to a lower voltage called the "maintaining voltage" "v". the bulb will continue to conduct current until the applied voltage drops below the "extinction voltage" "v" "(point d)", which is usually close to the maintaining voltage. below this voltage, the current provides insufficient energy to keep the gas ionized, so the bulb switches back to its high resistance, nonconductive state "(point a)".