Geissler tubes are gas-discharge tubes. There's a whole family of those - neon lamps, gas-discharge rectifiers, thyatrons, ignitrons, krytrons, etc. Those were the first electronic devices with significant power-handling capacity. All have some gas inside that can be ionized. They usually don't have a heated filament, and don't work by thermionic emission. They're definitely the ancestors of fluorescent light bulbs. As power devices, they were used in specialty devices such as lamp dimmers (rarely, but I've seen one), motor controllers (rarely, but done during WWII), and, of all things, centrally controlled school clocks (IBM/Simplex). Niche.
Then there were vacuum tubes. Their genealogy starts with the Edison Effect (put an extra element in a vacuum light bulb, and there's some current flow), and go on to Fleming's diode and then De Forest's triode. At last, gain! These were all low-power devices, but they could amplify small signals. They made radio, TV, and computers go before semiconductors.
Gas-discharge tubes and vacuum tubes aren't that closely related. They work on different physical principles. During the tube era, they often came in the same tube packages, so people think they're similar.
The 01951 book I learned digital logic from, by Dennis Ritchie's father and two of his Bell Labs colleagues, has a chapter on switching with "electron tubes, both vacuum and gas-filled," and "semi-conductors": https://archive.org/details/TheDesignOfSwitchingCircuits/pag....
(Fluorescent light bulbs, by the way, do have a heated filament, and do work by thermionic emission, though cold-cathode fluorescents like those used in old LCDs don't.)
The respective niches of vacuum tubes and gas switching tubes could be very crudely summarized as high speed and high reliability. Even primitive vacuum tubes had switching times in the microseconds, and by WWII it was below a nanosecond, like transistors, but they relied on hot filaments that eventually burned out. Cold-cathode gas tubes, by contrast, essentially never break, but they take close to a millisecond for the gas to deionize so they can stop conducting. They can switch higher-frequency signals, but they can't switch on and off faster than that. Keister, Ritchie, and Washburn say of hot-cathode gas tubes:
> The speed of response of the tube is contingent primarily on the ionization and de-ionization times of the tube. Depending upon the gas, the ionization time ranges from a fraction of a microsecond to several microseconds; the de-ionization time is ordinarily of the order of a hundred to a thousand microseconds, though lower values have been achieved. The tube, then, can respond very rapidly to input signals applied to operate the tube, but considerably more time must be allowed for extinguishing the tube.
When I first read this when I was eight, "a hundred to a thousand microseconds" presumably sounded incredibly fast, but of course it's painfully slow for computation. Of cold-cathode tubes, they say:
> Moreover, since the cold-cathode tube has no filament, no standby current is consumed. The speed of response, though somewhat less than that of the hot-cathode gas tube, is sufficient for most applications. The ionization time depends upon the time necessary to transfer the discharge from the starter gap to the main gap, and it is generally less than a hundred microseconds. Main gap de-ionization times are of the order of one to ten milliseconds.
You might hope that this would have improved since 01951, but, as far as I can tell, it never did.
They continue:
> Because of its suitability to switching circuits, the electron tube circuit examples contained in the remainder of the chapter are, in the majority of cases, based on the cold cathode-tube.
(They do, however, include a few vacuum-tube circuits.)
The rest of the book is about relays. Vacuum tubes and semi-conductors were, from their point of view, niche.
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