||When a high voltage is applied to electrodes sealed within a bulb containing an inert gas at suitable pressure, light is produced at the negative electrode, or cathode. For that reason this kind of light source is often referred to as the negative glow lamp. When operated on AC, the reversal of polarity is so rapid that both electrodes appear to glow, and this was capitalised upon in a tremendous variety of glow discharge lamps that were popular until a few decades ago.
Neon is the most common filling, this element producing the strongest emission for a given current, and having an efficacy of about 0.3 lumens per watt. Argon and helium filled versions were also offered as miniature UV sources. Since the amount of light produced is limited, there are few applications for the glow lamp in general lighting. They are however extremely dependable, and do not suddenly fail as is the case with most other light sources. Thus they proved to be valuable for use as indicators and pilot lights on instrument and control panels, where a failed lamp might cause confusing information to be sent. The larger neon glow lamps found applications primarily as nightlights, for instance in hospital corridors or children's bedrooms, where their extremely low current consumption made the cost of operation almost insignificant.
Glow lamps have a negative volt-amp characteristic and thus require a ballast. On account of the low current, a simple resistor is suitable. American screw-based lamps are almost always equipped with an internal resistor for 105-125V circuits. Mains voltage variations have relatively little effect, light output being almost directly proportional to current, while life varies roughly inversely as the cube of the current.