Let's See If This Sticks

I find it interesting that in electrical engineering, the concept of negative feedback is the used to prevent the output of a device from rising beyond it's capabilities or to reduce irregularities, but in the realm of people it has just the opposite effect. This is especially true when the form of negative feedback used in electronics, where a device feeds back upon itself, happens for a single person, unto themselves. 

Positive feedback, on the other hand, was once used to wonderful effect: the single triode regenerative radio receiver. In my opinion this simple circuit in many ways single-handedly sparked the electronic conceptions held by most people during the 20th century. The circuit below is entirely practical, and uses a #30 direct-heated thermionic valve, or tube. 

 

The circuit illustrates the use of positive feedback in such an elegant way, where small amounts of power "tickle" an electrical circuit into behaving as if its parts were made of impossibly perfectly ideal electrical components. Very small amounts of gain can be multiplied many times, and the filtering action of the tunable tank circuit becomes very sharp up to a point, where further increases in positive feedback result in oscillation, rather than simple amplification. The speaker was made from guitar pickup wire, a mints tin, soft steel wire, a few magnets, some bolts, paper, epoxy, solder, and tape. I based my design on plans for a simple home made magnetic diaphragm speaker published by H. P. Friedrichs in his book The Voice of the Crystal. I highly recommend both his currently published books, and I look forward to his next.