Gas filled tubes

Apart from early deliberately soft valves (tubes) from about 1905 to 1918, some types  are only possible by having low pressure gas.

Indicators and Lamps

The earliest kind were decorative Geissler tubes invented in 1857.

Medium to high power can be mercury vapour, a heater is required to ensure some mercury is a gas. Used in street lights, UV lamps, projectors. Common domestic type are the Florescent Tube, CFL and some kinds of LCD backlights.  LCD backlights can also be cold cathode or LED. A phosphor coating converts the UV light and some of the plasma hitting tube walls  to visbile light.

Small indicators can use hydrogen, argon, neon, xenon or krypton.  Xenon is also used for  high power lamps. Many “neon” signs are actually Mercury Discharge types with coloured phosphors. Small Xenon tubes were displaced by white LEDs on camera flash, but larger camera flash and projectors still use Xenon.

Information Indicators

These usually are Neon, but Argon and Hydrogen is also used. These include Nixie (each digit 0 to 9 is an electrode), Planaplex (7 segment), bargraphs, dot matrix panels (Plasma Displays) and counters used as numeric displays.

Switches and Rectifiers

Mercury vapour  devices can be fabricated to rectify or switch 1000s of amps at 1000s of volts. Neon or other gasses can be used in a triode construction without a heater (low power cold cathode) or with a heater for high power.  The operation can be very like a Thyristor (also called a Silicon Controlled Rectifier). A triode requiring trace gas for operation is a Thyratron.

Oscillators and Counters.

A simple neon indicator with parallel capacitor and series resistor will form a relaxation oscillator. A Thyratron (triode with neon or other gas) can be a triggered or synchronised oscillator. Two kinds of counter in a single tube were developed in the 1940s, the Trochotron (Vacuum tube)  and Decatron (or Dekatron, Neon, Argon  or Hydrogen). The Dekatron was used as both the counter and display in instruments as an alternative to separate counters driving Nixie. With each pulse the glow travels to the next pin. Specialist types had 6, 8, 12  or 16 etc display/counting elements rather than 10.  The Trochotron required also a magnetic field.


These look like filament lamps and glow a bright orange. They can use a lamp base (screw or bayonet) or a regular tube base. The wire is actually iron and the glass envelope has low pressure hydrogen. They act almost as a constant current source as the  resistance rises rapidly with temperature.  They were replaced by thermistors and also tapped wirewound resistors when Mains voltage supplies became more stable.

Surge Arrestors

These are sometimes still used. In principle very similar to a neon indicator in construction. Traditionally used on telephone lines and radio aerials to limit the high voltage pulse from a nearby electrical discharge such as lightening. Zinc Oxide Semiconductor discs are often used now.