Nixie tubes are electronic boxes to display the numbers 0-9 just like your alarm clock, but using bent wires in the shape of numbers. It’s like having a display back then on a mechanical device, having electronics visible in a glass tube. You could say neon lamps, although not exactly. They have numbers sealed within the tube, separated from each other. (https://en.wikipedia.org/wiki/File:Inside_the_nixie_tube_vnutri_u_nei_neonka.jpg)
Introduced in 1955 and made by a vacuum tube manufacturer, Nixie is a trademarked cold cathode neon readout tube. It was originally known as NIX I (Numeric Indicator eXperimental No. 1). It was later displaced by VFD, LED and LCD displays. It looks like a vacuum fluorescent display but operates in a different manner. Unlike the vacuum display, Nixies have no heater or control grid, usually a single anode The glass contains multiple cathodes and the number-shaped wire-mesh anodes.
The tube is made up of single anode (positive terminal) which is the shape of mesh around the outside. Cathodes (negative terminal) have the shape of number or characters to be indicated. Multiple cathodes are used to display multiple characters or numbers. These cathodes in Nixie tubes are separated from each other using tiny ceramic spacers. The anode wire-mesh is wrapped around the stacked numeric cathodes. This single anode serves as positive terminal for all the cathodes.
The most common form of Nixie tube has 10 cathodes used for displaying numerals 0-9, some also include a decimal point or two. The numbers are arranged in a stack, one behind the other, so each character appears at a different depth when glowing. Applying power to cathode of this cold-cathode tube surrounds it with an orange glow discharge. The tube is mostly filled with neon gas at low pressure, and a little mercury or argon. It operates under 40%C (104F) at a room temperature. The glowing plasma is formed by passage of electric current through low pressure gas. The gas used is Neon gas, and Mercury is added to lengthen the lifetime of tubes and also adds blue or purple tints to the color.
Nixie used a low-current (a few milliamps) at 170V DC between cathode and anode to get typical red-orange color. Nixies exhibit negative resistance and will keep glowing up to 30V below the strike voltage. The digit sequence is usually 6 7 5 8 4 3 9 2 0 1; 6 in the front and 1 in the back.
In early days of digital technical equipment like Voltmeters, frequency counters, etc Nixies were used as numeric displays. They were also used in early desktop calculators, and on elevators to display floor numbers.
The earliest Nixie tubes lasted around 5,000 hours and later improved ones to 200,000 hours. Nothing other than mechanical failure has been formal definition of what brings Nixies to end of life. Failure of a Nixie tube could be due to increased striking voltage causing flicker or complete failure of light, open or short internal circuit due to physical abuse, cathode poisoning preventing some or all characters from illuminating. Using Nixies at extreme electrical conditions, beyond specified parameters, especially excess current, will accelerate their demise. If you make a project with them be sure to be cautious on your power supplies! The Nixie tube based clocks that are available now, last for over 30 years.
The Nixie was not only the device for displaying numerals or other information using glow discharge. Other firms had used names like Digitron, Inditron, and Numicator for similar tubes although Nixie has became popular in retrospect. A similar device called pixie tube used numeral-shaped holes instead of shaped cathodes was in existence in the days of Nixie.
After Nixie Tubes:
Sadly for Retro Future fans, in the 1970s, light-emitting diodes (LEDs) and vacuum fluorescent displays (VFDs) superseded the Nixie tubes. VFDs required a much lower voltage to operate (5V vs 150V+) which made it easier and cheaper to use. LEDs used a the same low voltage that IC chips used, and were also much smaller and sturdier without the fragile glass tube. This made LEDs the prefered choice in digital watches and pocket calculators, which would not have been practical with Nixie tubes.
Nixie Tubes in current time:
Nixies were too expensive to be used in mass-market goods such as clocks. They are not produced anymore. Recently the demand for Nixies is surging and causing a price hike. Apple inventor Steve Wozniak even has a Nixie tubes based watch (https://en.wikipedia.org/wiki/File:Nixie_Wozniak.jpg)
Where to get:
They are not cheap, and are getting more expensive because they are becoming more rare as they are no longer actively produced. You can find parts and clock kits on eBay , or a fully constructed nixie clock at Amazon.
Here is a video of someone unboxing one of the modern kits. Near the end it shows Nixie tubes in action!