A nixie tube, also known as a glow tube, is an electronic device that displays digital and other information. The glass tube consists of an anode and a plurality of cathodes made of wire mesh. Most nixie cathodes are in the shape of numbers. The tube is filled with a low-pressure gas, usually mostly neon with some mercury and/or argon. When one cathode is charged, the nixie tube emits a colored light, usually orange or green, depending on the gas inside.

The most common form of nixie has 10 cathodes in the shape of the numbers 0 through 9, and some nixies have one or two decimal points. However, there are other types of digital tubes that display letters, marks, and symbols. For example, the cathode of a “digitizer” is a mask made of a template with holes in the shape of a number.

A DC voltage of 170 volts is applied between the cathode and the anode, and each cathode emits the red-orange glow of neon. Different types of nixie tubes have different colors due to different gas mixtures. Long-lived digitizing tubes are manufactured with mercury, which reduces sputtering, and the resulting light is in a blue or purple tone. In some cases, these colors are filtered by a red or orange filter coating on the glass.