Current rating and life of cold-cathode tubes

A COLD cathode tube is a gas discharge device capable of serving as a relay, a rectifier, or a voltage regulator.^1,2 It depends for its action upon the properties of gas discharges. The salient property employed is the fact that there is a difference between the breakdown voltage, or voltage necessary to initiate a discharge, and the sustaining voltage, or voltage drop across the tube when conducting. The principles involved are not new but a practical device capable of operating on voltages of 150 volts or less has awaited the relatively recent development of suitable cathode coating materials. The cathode surface employed at present consists of a coating of barium and strontium oxides applied to a nickel base. These oxides are partially broken down to barium and strontium metal during exhaust. During the operation of the tube the barium is continually removed from the cathode by sputtering and is replaced by further reduction of the oxide reservoir. The life of the tube is limited, therefore, by the amount of material present and by the rate of sputtering.