Differential

Differential negative resistance fascinates me, and I recently read about a supposed "true" negative resistance device. The experiments showed IV curves with straight negative slopes that crossed the origin. At first I thought I had stumbled upon a neat home-brewable device for simple two-port amplification experiments, but things just didn't look right. The schematics indicated the device had four ports, and they were measuring voltage on one set and current on the other, which is wrong as there is significant semiconductor material between the voltage ports and the current ports in the form of carbon fiber. The origin of the work, a university website, looks fairly legit:
http://www.wings.buffalo.edu/academic/department/eng/mae/cmrl/Apparent%20negative%20electrical%20resistance%20in%20carbon%20fiber%20composites.pdf
 So I paid more attention, hoping that this could be a useful device for homebrew electronics, but the more I thought about it the more this thing seemed to be, at best, a peculiar inverter-attenuator. Any negative slope in the IV curve can be used to amplify, sometimes fairly linearly. But, this device has no possible way of developing gain, and furthermore if the bias or physical pressure on the device is carefully adjusted the "resistance" thus measured by their methods appears to cross through zero! Is this thing a room temperature superconductor? I'm pretty damn certain it's not. The authors, despite supposed scientific expertise, have made a freshman error in their methods. What they are measuring is NOT a resistance.

Things look better

If viewed in the right light, everything can seem better. One such light is that bright late-afternoon sun that streams in through the windows and gently warms the belly of a tired engineer lounging on his couch.

Yay seasonal happiness!

Something else that is as reliable as nice weather in summer is the happiness of a child. I made a kit of this simple crystal radio for a friend's nephew. Made of coils of wire on the plastic case that once held a stack of blank CD-R discs, it's almost magical in an age of hyper-dense electronics. I think it's amazing that such a simple arrangement of parts works well enough to be useful as an AM radio. This design is more sensitive than selective, but that can be affected by different arrangements of the components. One of the nice things about "passive" circuits like this is that there is no harm done by hooking it up wrong.

With a proper antenna and relative proximity of the transmitter, FM may also be received by "passive" means, albeit very weakly.

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.