Nixie Tube
In a Nixie Tube display each numeral is a complete, lighted cathode in the shape of the numeral. The cathodes are stacked so that different numerals appear at different depths, unlike a planar display in which all numerals are on the same plane relative to the viewer. The anode is a transparent metal mesh wrapped around the front of the display. The tube is filled with the inert gas neon with a small amount of mercury. When an electric potential of 180 to 200 volts DC is applied between the anode and any cathode, the gas near the cathode breaks down and glows. The digits glow with a orange-red color.
The name Nixie came about accidentally. A [Burroughs] draftsman making drawings of the device labeled it NIX I, for Numeric Indicator eXperimental No. 1. His colleagues began referring to it as "Nixie," and the name stuck (Scientific American, June '73, pp. 66).
Interestingly enough the Nixie design is considered "failsafe". If a filament (cathode) fails, the numeral is not illuminated. Whereas, in a seven-segment display if one segment fails, a number other than the intended number may be displayed. The Casio 121-A and the Sharp QT8-B used Nixie tubes.