A neon lamp is a gas discharge lamp containing primarily  gas at low pressure. The term is sometimes used for similar devices  filled with other noble gases, usually to produce different colors.

A small electric current, which may be AC or DC, is allowed through the tube, causing it to glow orange-red. The exact formulation  of the gas is typically the classic Penning mixture, 99.5 percent neon and 0.5 percent argon, which has lower striking voltage than if its pure.  The applied voltage must initially reach the striking voltage before the lamp can light. Once lit, the voltage required to sustain  operation is significantly -30 percent lower. When driven from a DC source, only the negatively charged electrode or cathode will glow.  When driven from an AC source, both electrodes will glow each during alternate half cycles. The lamp operates using a low  current glow discharge. Higher power devices, such as mercury-vapor lamp or metal halide lamp use a higher current arc  discharge.

Once the lamp is lit, a  lamp has a negative resistance characteristic increasing the current through the device increases the number of  ions, thereby decreasing the resistance of the lamp and allowing even more current. This behavior occurs between the points  labeled A and B on the lamp current vs. voltage graph.  Because of this characteristic, electrical circuitry external to the   lamp must provide a means to limit current through the circuit or else the current will rapidly increase until the lamp is  destroyed. For indicator-sized lamps, a resistor is conventionally used to limit the current. Larger sign sized lamps often  use a specially constructed high voltage transformer or ballast to limit the available current, usually by introducing a large  amount of leakage inductance in the secondary winding.

When the current through the lamp is lower than the current for the highest-current discharge path, the glow discharge may become  unstable and not cover the entire surface of the electrodes. This may be a sign of aging of the indicator bulb, and is exploited  in the decorative ..flicker flame..  lamp. However, while too low a current causes flickering, too high a current increases the  wear of the electrodes by stimulating sputtering, which coats the internal surface of the lamp with metal and causes it to darken.