An interesting little electrometer device.

[Richard Hull]

At a hamfest in August last year, in a junk box marked anything this box 25 cents, I found this neat little glass tube that looked unusual. I dropped my quarter in the "honor box", (the table owner was probably off looking for goodies, himself). Anyway, I forgot about it and only recently tested it's characteristics. It turns out it is some sort of gas filled vacuum tube that is designed to detect tiny changes in an electrostatic field. The gas has to be neon and when it fires the tube glows orange red in the hollow cathode! I attach photos and a diagram of the tube and how I set it up to fire or trigger when a charged teflon rod was wiggled at a surprising great distance.

All old NE-2 and other classic neon indicator lamps have a rather huge turn-on, turn-off hysteresis. They require 70+ volts to turn on, but can often stay lit down to 58 volts. It turns out the needle like trigger electrode in the tube can, in a changing field, with an attached isotropic capacity loading, act to detect and indicate 10e-15 amp currents!! (verified with a Keithley electrometer in my lab.). the assumption is that field emission from the hyper fine needle tip acts like a grid to fire the critically setup tube. (see diagram)

The diagram and images are pretty self-explanatory for the adroit here. As noted, this thing is a bear to set or make ready for hyper sensitive changing electric field detection. Once set, just moving near the device can create enough change in the local field to trigger the tube into glow mode. I demonstrated this at the local HEAS monthly meeting recently. I admonished all to stand perfectly still as I crept away to get my teflon rod. I decided to stand back about 8 feet and rub the rod with my felt cloth to charge it first before approaching. After two strokes of the cloth, someone yelled out IT'S LIT!! Boy is that puppy sensitive.

If you mange to find one of these little tubes at a hamfest, pick it up! Cool stuff can often be found in the 25 cent and 50 cent junk boxes that often abound on tables at hamfests.

Richard Hull

[Rich Feldman]

Wow, that looks just like the middle of a 1960's-era F5B smoke detector head.
Even the yellow and red flex wires from tube base.
I wonder if you got somebody's smoke detector part, or if the smoke detector was designed around an existing electrometer device.
Is your hollow black chamber radioactive, with two little wires "riveted" inside the cylindrical wall? Don't sleep next to it!

.
I just got an F5B to detect smoke, with no semiconductor electronics inside or out, including adjustable power supply much like yours.
Wired up a couple of external resistors and a NE-2 glow lamp, which switches on when smoke is present.
Details to follow when my photos and diagram are more complete.

I found the bare tube to be triggerable in either direction, with different performance, like a bipolar transistor with E and C interchanged.
For first and only all-up test, which was successful, my bias polarity was opposite to yours.

Like modern smoke detectors, F5B has two ion chambers in series, made slightly conductive by alpha particle irradiation, forming a voltage divider. (no high-meg resistor needed)
Their common node is connected to cold cathode trigger electrode.
Round plate inside the black chamber (non smoky) goes to red wire & recessed cup electrode, which I made positive. Hollow anode?
Outside can of the smoky chamber was connected to negative side of power supply, and (via my external resistor) to the yellow wire and mesh electrode inside trigger tube.
Smoke increases the positive voltage on trigger with respect to mesh electrode, and it worked for me.

More later.

[Cai Arcos]

Richard:

I will eagerly await a more detailed description of the very interested device you have presented now!
In the original FAQ post I quoted wikipedia for an invention date 1939. However, the best I found was this patent from 1962 with apparently no references:

https://patents.google.com/patent/US3353170

It also mentions the double chamber arrangement you mention.

[Rich Feldman]

See also US patent 3160866. Its Figure 2 shows dual chamber and trigger tube, with a resistor between anode and smoke-free chamber terminal. With my F5B unit the R needs to go between cathode and smoky chamber terminal. Triggering seemed to be well behaved when total voltage was between about 230 and 320 volts.

[Rich Feldman]

Richard would've noticed if the black chamber on top of his trigger tube were marked as radioactive.
I will guess that it, or same-size part in smoke detector, is a derivative product.
Anybody know which came first?

Of the F5B's nominal load of 80 uCi, anybody know what fraction is in the sources that irradiate the inner chamber?
Perhaps this discussion belongs in a new thread under Radiation Detection.

[Richard Hull]

Like I noted, this was a little tube item I found in a junk box at a hamfest. I did note the warning on the black dome, but read no radiation off of it once I got home. The tube was the thing that drew me in. The tabs on the inside of the hollow aluminum shell that you show in your image were missing. What you see in my images was what I pulled out of the junk box at the hamfest. I rooted around in the box to see if there were any more, but to no avail.

The big aluminum dome and its connection to the needle electrode tended to draw me to the electrostatic nature of the device, having used such isolated, isotropic capacitances in a number of my electrostatic experiments in the past and the advantage gained by using toroids back in my Tesla coiling days to distribute the high field and store energy prior to release.

This is just too cool. I know that many of the NE-2 and other neon lamps did have a small amount of radioactive gases in them to reduce the "dark effect" (this info was from the larget 1964 GE neon lamp handbook that can be downloaded on the internet). I checked and my GM counter detected no radiation from the tube or the aluminum shell, inside or out.

Richard Hull

[Rich Feldman]

Yes, very cool. Hard to keep away in order to take care of day job and family obligations.
Here's a work in progress: F5B sectional view drawn from measurements.
The yellow dots represent alpha particle sources. Blue box is envelope of trigger tube.
Who can find documentation, even a model number, for the nonradioactive device Richard found in bin at ham fest?