If you want to build your own vacuum tubes, whether amplifying, Nixie or cathode-ray, you’re going to need a vacuum. It’s in the name, after all. For a few thousand bucks, you can probably pick up a used turbo-molecular pump. But how did they make high vacuums back in the day? How did Edison evacuate his light bulbs?
Strangely enough, you could do worse than turn to YouTube for the answer: [Cody] demonstrates building a Sprengel vacuum pump (video embedded below). As tipster [BrightBlueJim] wrote us, this project has everything: high vacuum, home-made torch glassware, and large quantities of toxic heavy metals. (Somehow [Jim] missed out on the high-voltage from the static electricity generated by sliding mercury down glass tubes for days on end.)
The pump itself is very simple. Drops of mercury catch bubbles of air from the vessel to be evacuated and eventually all that’s left is vacuum, at least until the mercury starts boiling. [Cody] even experiments with cooling the mercury in liquid nitrogen to lower its vapor pressure and get an even better vacuum. It looks like it succeeded, but with his rudimentary measurement technique we can’t be sure.
Anyway, if you’ve got a few dollar’s worth of glass tubing, a few kilograms of mercury, more than a few hours to wait, and the muscles to lift the mercury up to the top of the tube, you can build yourself a vacuum pump that’s perfectly adequate for making light bulbs or Nixie tubes. [BrightBlueJim] suggests updating this project from mercury to something like gallium that’s less toxic but still really dense. We like that idea.