A while back, I bought a big, weird-looking tube on eBay. I had more-or-less forgotten about it until I came across it again whilst moving house.
Curious, I endeavored to determine what it was. After some research, I found out it was a
"finebeam tube" (Fadenstrahlröhre).
In fact, my tube looks like the one in the picture, and bears the same part number. However my unit sports an oddball plug instead of the ordinary pins on the (supposedly more modern one) available for sale today (for a whopping US$1,858!).
The tube contains a simple electron gun, a set of deflection plates, and gas (generally helium, hydrogen, or neon) at very low pressure, enabling the beam to be observed. It's used to illustrate electron physics, the effects of electrical and magnetic fields on electron beams (with an additional set of Helmholtz coils and a variable power supply), and even compute the specific charge of an electron.
Some searching failed to reveal a socket for it, or even a pinout. There are only six pins, and their functions were reasonably well documented (heater, heater/cathode, Wehnelt [focus] cylinder, anode, and two deflection plates). Probing the connector, I found two pins with continuity. This showed that the tube was at least electrically intact, and those would be the heater pins. The various tubes on the web seem to want between 4 and 7.5 volts on their heaters. May as well try to light it up! I rounded up a tube tester, and wired a couple of its socket pins to the heater pins on my tube. Gradually bringing up the heater voltage in a darkened room presently resulted in a dim orange glow.
Encouraged, and motivated by the possibility of actually seeing an electron beam with my own eyes, I rounded up an adjustable regulated power supply (Heathkit IP-32) capable of delivering zero to 400 volts. The procedure was a bit tedious, as I'd have to connect a wire to a prospective pin on the tube, turn on the supply, turn off the lights, and wait for my eyes to adapt to the darkness to see if there was a visible beam.
( I got a beam! )
It was dim and fuzzy, but definitely there! I brought fizzygeek down to see it, and she was able to see the beam easily (I suspect her years of darkroom experience helped).
I wondered if I could photograph it. I grabbed a tripod and borrowed fizzygeek's Canon G9, as it has a reasonably fast lense and sensitive (ISO 3200 equivalent) sensor, but no joy. No worries, just an excuse to get out the big guns. In this case, a DSLR and a cable release. Unable to get good focus with a generic 55mm lense, I got an ancient Micro-Nikkor I had picked up on eBay for $15 a few years back. Now to figure out exposure. I generally have a good eye for such things, but this was a bit far out on the reciprocity curve. I tried a few seconds, but just obtained a faint smudge that I assume was the glowing cathode. Cranking the camera all the way to its 30-second limit, I still didn't have much. Then I realized I hadn't checked the aperture! Sure enough, it was pretty stopped down, so I cranked it all the way open. Bam, bright screaming overexposure! This surprised me a little, as I didn't think there was that much light available, but I happily tweaked settings until I got a decent exposure. I took the memory card upstairs to view the results on the big calibrated LCD monitor.
I figured the next thing to try was to deflect the beam using the built-in deflection plates. The power supply included an additional "bias" output, variable from zero to -100 volts, so I strung another wire, and started trying pins. Sure enough, I was able to deflect the beam. Since electrons are negative, and I was applying a negative voltage, the beam deflected away from the plate I was connected to. This way, I was able to figure out which pins went to the deflection plates.
( pictures of a bending electron beam )
I noticed that the anode cone was glowing in the pictures. Very pretty, but I didn't remember a glow. The purplish cast to the photo gave me the missing clue: that particular DSLR was the UVIR version (thanks gravitrue!), sensitive to ultraviolet and infrared light as well as the visible spectrum. And I hadn't moved the UV+IR blocking filter when I changed lenses.
I went back downstairs, screwed on the filter, and tried again. Sure enough, the exposure was now about what I expected (the additional light outside the visible range was what had thrown me off). And indeed, in the visible-only exposure, I didn't see the anode glow, and the heater glow was a more appropriate intensity and dull orange colour (the copious IR had made it unnaturally bright, and tinged it an otherwordly lavender).
( more pictures )