An auxiliary electrode that is used in a glow-discharge tube (see gas-discharge tube) in order to initiate the glow discharge; it is also used in an arc-discharge tube to initiate the arc discharge. A sufficiently high potential difference is applied between the starter electrode and the cathode and once the glow discharge is established the necessary voltage is applied to the anode. The maintaining voltage is considerably lower than the voltage required to initiate the discharge and this method of operation protects the main circuit from excessively high voltages. The starter electrode may also be used to maintain the discharge during a period when the main anode is ‘off’.