Regenerative Shortwave Radio Receiver - homebuilt experimental design, uses one 6SN7, a dual-triode vacuum-tube, and a design that uses regenerative feedback. Despite being an ugly hack, it works remarkably well. I was able to listen to Tokyo (and WTWW Nashville, Tenn. :) ) last night. See Regen. Radio tab for schematic. Feedback circuits & processes are interesting.
The regenerative circuit is very interesting - if you look at the schematic, you can see that the 6SN7 is basically two triode tubes in one bottle, and that the radio signal that is detected by the first triode, and fed to the grid of the second triode to be amplified, is then also sent back to first triode, thru the 47K potentiometer. The pot. acts like a volume control, except when you turn it up too high, you drive too much feedback thru the signal detector, and you get a "motorboat" sound, or a howl or a squeal. Back off the gain a bit, and typically, a nice, boosted signal of the distant station can be heard well.
This careful tuning with sensitive feedback process is very much like what happens when the laser-rail of the TEA Laser are adjusted, which is why I suspect there is a feedback process happening in the lasing of the N2 molecules.
The whole conceptual idea of a feedback process is very interesting - it is also very much the basis for both computer-based neural-networks, which use calculus methods to back-propagate small changes to neural node weights - and also probably how the real neuron state potential is adjusted in bio-systems (like in our own brains).
You can "suss-out" the degree of reflective (reflexive?) feedback in market activity, and you can gain some true, actionable insight into how prices are being set in a competitive marketplace situation.
I find that actually constructing the physical devices which make actionable use of these process methodologies, allows one to gain unique insights that otherwise often do not become apparent.