If you’ve got a digital alarm clock (or a timer on a microwave oven, VCR, or DVD player), chances are it has blue-green numbers (in which case, it’s using a vacuum fluorescent display, VFD) or red ones (which means it’s made from light-emitting diodes, LEDs). Either way, you’ll notice that it displays each of the digits in the time (10:30 or whatever) by lighting up a pattern of seven quite separate bars, commonly called “segments.” You can write all the numbers 0–9 with a seven-segment display and quite a few letters and words as well.
Nixie tubes also display the numbers 0–9 but in a completely different way. Look closely at a nixie tube and you’ll see it has the ten decimal digits made out of bent wire and arranged in a stack, one in front of the other, inside a sealed glass bulb. Underneath the bulb, there are lots of electrical contacts. Wire these up to an appropriate electronic circuit and you can make the numbers count up in sequence, tell the time, or do all kinds of other neat things.
Nixie tubes work a lot like neon lamps (although not exactly like neon lamps, as we’ll see in a moment).
The bent metal wires that display each number aren’t filaments, like the ones in an incandescent lamp. Each of them works as a separate negative terminal (cathode) in what is effectively a gas-discharge tube—so one nixie tube has ten cathodes. The cathodes don’t actually touch one another but are kept apart by tiny ceramic spacers. There’s also a single positive terminal (anode), shaped like a mesh or grid, that’s wrapped around the stack of numeric cathodes and serves all 10 of them.
Nixie tubes are very similar to neon lamps. Both look a bit like cathode-ray tubes (old-style televisions), in which electrons boil off a hot metal cathode at one end and race down the tube toward a positively charged anode at the other. But in neon lamps and nixies, the cathodes remain relatively cool (tubes like this are described as “cold cathode,” even though they’re generally on the warm side—about human body temperature) but the gas mixture that surrounds them is at a very low pressure (perhaps 1/100th of normal atmospheric pressure or even less—so typically less than 1000Pa or 0.01 atmospheres). When a voltage of about 170–180 volts is applied between the anode and one of the cathodes, the low-pressure gas becomes ionized (its atoms or molecules are turned into positively charged ions and negatively charged electrons). When electrons, ions, and atoms collide with one another (and with metal atoms ejected or “sputtered” from the cathode), we get a glowing, fuzzy “coating” of light forming all around the cathode, very close to it, that follows its shape precisely—effectively making it appear as though one of the numbers 0–9 is illuminated. Applying a voltage to a different cathode makes a different digit “light up.”