Gas discharge tubes are sealed glass tubes containing a pair of electrodes and filled with a gas that only conducts when a high voltage is applied across the electrodes. When a voltage spike appears across the device the gas is ionised (becomes a conducting gas plasma) which short-circuits the spike energy away. Gas discharge tubes are often used to protect sensitive circuit inputs from potentially damaging interference voltage transients. The large amount of energy that can be dispersed by gas discharge tubes makes them a good choice for lightning surge protection, particularly for telecommunications equipment.