One of the things I find fascinating is nixie tubes. A nixie tube is an electronic device used for typically displaying numerals (although practically anything can be displayed). Nixie tubes were originally manufactured in the 1950's primarily for use as displays in electronic equipment. Although it resembles a vacuum tube in appearance, its operation does not depend on a heated cathode, and is therefore sometimes referred to as a cold-cathode.
Unfortunately technology has moved on, and nixies have been superseded by VFD (Vacuum fluorescent display) and subsequently LED (Light-emitting diode) displays. And as such are becoming harder to obtain. As they are no longer manufactured, the only supplies available are NOS (new old stock). Items that have never been used, but have been sitting in storerooms for many years.
One of the largest Nixie Tubes still available is the IN-18 (pictured below). The digits are 40 mm high by 18 mm wide.
(Front view of an IN-18 Nixie Tube)
(Side view of an IN-18 Nixie Tube)
You should be able to see in the two pictures how the individual digits are stacked on top of one another.
One of the more popular uses for Nixie Tubes today is as displays for clocks. But more on that later....