MAGIC EYE DEMONSTRATION
Representative circuit for magic eye tube display.
Shadow control varies target display by variation of -ve grid voltage.
When switched to "shadow + flick" neon relaxation oscillator of approx 1Hz causes magic eye to flick.
The green glow you see on a electron-ray tube is produced when electrons emitted by the cathode strike the "Target" (anode or plate) which is coated with the fluorescent mineral willemite.
Chemically, willemite is known as zinc orthosilicate: Zn2SiO4.
Worldwide, willemite is a relatively rare mineral. As luck would have it however, economically significant deposits exist at the Franklin and Sterling Hill mines in New Jersey.
This proximity to Dr. Du Mont’s lab no doubt influenced his use of the mineral.
Had other members of the fluorescent mineral family been used, the glow of tuning-eyes (as well as early cathode-ray tubes) could have come in shades of yellow, blue, or red.
Eventually, electron-ray tubes were produced using a manufactured chemical equivalent to willemite - the same "P1" material used in oscilloscope tubes.
Target dimness is believed to be due to physical or chemical contamination of the willemite boundary layer by Barium and other ions that are boiled off through normal cathode emission.
This contamination is visible as streaks of darker gray discoloration to the otherwise light gray coat of a new electron-ray tube target.
If you’re rummaging through a box full of tubes at a flea market and spot a tuning eye tube, look for this discoloration before you buy.
Be certain to examine the target surface in a bright light.
The target of a good - or reasonably good - tube will have a consistent light gray tone.
Look for two thin lighter gray streaks that are in the same line but opposite one another.
These light streaks are produced by the shadow cast by light shield’s supporting pins and are evidence of a well-used tube
Gas counting tubes.
Sometimes called "glow-transfer" counting tubes, or Dekatrons (Ericsson's Trademark), these noble-gas-filled counting tubes are based on plasma/arc strike/break hysteresis physics.
In adddition to counting, the internal electrodes glow visibly, and hence are arranged to be visible through the end (top) of the glass envelope, to show the counters current value.
It is common for multi-decade counters and scalers to have the counting tubes poke their heads out of the front panels, each digit's visible "bits" labelled 0-9.
Various electronic circuits shape the pulse(s) to be counted into overlapping pulses that "bump" the plasma arc from one electrode to the next (hence the "glow transfer" from electrode to electrode).
Counting is done by picking off a signal from the Nth (generally 10th) electrode; an output pulse is therby generated every N (10) pulses.
(In fact most had two or three electrodes per digit-bit to perform the counting, but the general principle is the same.)
Each counting cycle takes anywhere from hundreds of nanoseconds to tens of microseconds to complete, since gas ionization is a slow bulk process; 100,000 counts per second is considered fast, a million pulses per second is about maximum.
The IBM Electronic Statistical Machine, Type 101 for tallying punch-card results, allegedly contains three-phase tubes.