In 1675, while transporting a barometer by night, the astronomer Jean Picard noticed a glow inside its glass tube, just above the mercury. As the mercury sloshed and splashed across the surface of the glass, a static electric charge had built up, which was discharging by ionizing the residual gas molecules inside the evacuated tube. [Styropyro] recreated this effect, and found that the dim glow could be made much stronger by adding some noble gas to the tube.

It starts with a simple recreation: he took a volumetric flask, attached a narrow glass stem to the mouth, added some mercury to the flask, evacuated it with a vacuum pump, and sealed off the glass stem. This produced a faint glow when shaken, but it was only really visible under very low light. When [Styropyro] brought it near a Tesla coil, however, it did glow much more brightly.

Backfilling an identical flask with neon to about 40 millitorr produced a much more spectacular result (a low pressure in the tube is necessary, but moderate pressure variations don’t significantly alter the effect). When shaken even slightly, this neon-containing flask produced a bright orange-red glow just above the surface of the mercury. Points of obstruction, such as those in a zig-zag tube, produced a brighter glow. A krypton-containing tube glowed blue, but less brightly than the neon tube.

Since this is, essentially, a triboelectric effect, other materials besides mercury should work; [Styropyro] tested several materials, and found that pieces of Teflon produced a faint glow, and copper beads a somewhat brighter glow. Unfortunately, Galinstan, the obvious replacement for mercury, wets and coats glass, preventing a charge buildup.

Without an added noble gas, the standard glow of barometric light comes from the excitation of mercury vapors, a glow which can also be seen in mercury rectifiers, and which excites the phosphors of fluorescent light bulbs.

Thanks to [Vik Olliver] for the tip!

Second Reality is a legendary demoscene release by Future Crew, which won Assembly 1993 with its technical and artistic mastery. [Niv Singer] decided to give the classic demo a spin on a rather unconventional sound system with a particuarly techy twist.

Hard drives are great for storing data. They’re designed for this purpose. What they’re not designed for is acting as speakers, but you can hack them into acting that way if you’re so inclined. For this project, [Niv] pulled apart a whole stack of drives, so they could be repurposed in this way. The principle is simple enough—just feed audio to the coil driving the head, and it will vibrate and wiggle around, creating soundwaves in the air. It’s not particularly effective, and you get limited volume with a terrible frequency response, but that’s half the fun. [Niv] actually took some of this into account, too. Four Western Digital Caviar 500GB drives were chosen for this build, two for the left channel, and two for the right. Each channel had a crossover, allowing one drive to handle low frequencies while the other handled higher ones. For a further nice touch, the platters spin with the beat as well, with [Niv] providing a great explanation on how this was achieved with the use of some nifty PWM tricks.

Files are on Github for the curious. We’ve featured plenty of hard drive speakers before, too. Video after the break.