Gas disharge plasma displays are the most well known type of alphanumeric displays.Their operation is based on the emission of light in a cold cathode gas filled tube under breaking down condition.These cold cathode numerical indicators are called Nixies.(Numicators and Numbertrons).
The Nixie tube is a numeric indicators based on glow disharge in cold cathode gas filled tubes.It is essenntially a multicathode tube filled with a gas such as Neon and having a single anode as shown in fig.
Each of the cathodes is made of a thin wire and is shaped in the form of characters to be displayed,for example,numerals 0 to 9.The anode is also in the form of a thin frame.In its normal operation ,the anode is returned to positive supply through a suitable current limiting resistor,the value of the supply being greater than the worst-case breakdown voltage of the gas within the tube.The gas in the vicinity of the appropriate cathode glows when the cathode is switched to ground potential.
Since 10 cathodes have to be associated with a single anode inside the glass bulb,they have necessarily to be stacked in differnt planes. This requires different voltages for differnt cathodes to enable the glow discharge.
Many Nixie tubes also possess dot-cathodes either on the left or right of the character to serve as decimal points.