I recently repaired the video board on my 54600A oscilloscope, which unlike my other scope, has an XY display mode and so can be used as a curve tracer. See below for a very simple front-end circuit needed to make this happen.You'll have to invert channel 1 to get the plot in the right quadrant(s).
I decided to take a look at some various cat's whisker radio detector minerals when excited with either AC or pulsed DC. I was able to observe negative differential resistance in several materials. This includes flame-treated galvanized steel, iron pyrite, and even galena! Any of these could be put to use as an amplifier or oscillator. There were also various different curves produced that had more ordinary diode characteristics, some bi directional, some not.
This is a healthy response from a piece of flame-treated galvanized steel probed with a sharp point of copper. Copying Nyle Steiner, I heat a piece of the metal one one side till it's red hot and throwing off sparks from the other side, and look for active spots on the side not hit with the flame. The above curve is displaying at 1V/division horizontal, and 10mA/division vertical.
One of my home made detector stands. Right now it is loaded with a hunk of iron pyrite, and the whisker is made from phosphor bronze. The next two IV curves were captured from this setup.
This one is quite obviously useful for amplification. Applying AC made it much harder to find a stable example of this characteristic. With DC excitation only, it was fairly easy. 500mV/div horizontal, 10mA/div vertical
This is a more typical response for the iron pyrite, bidirectional diodes with a forward voltage of about 2.5V
This is one of the curves from phosphor bronze on galena. There appears to be a backward leaning curve that might be useful for sustained oscillations. 100mV/div horizontal, 1mA/div vertical
Again, an interesting apparent negative slope region from galena probed with phosphor bronze. 50mV/div horizontal, 1mA/div vertical
I could also generate very weird curves like this one, which while not showing a clear negative slope region certainly implies that one must exist, even if it is a very abrupt transition. Galena is a more interesting material than I suspected. 50mV/div horizontal, 1mA/div vertical
While once again researching the topic of homemade transistors, I have
come across references to some people getting continuous wave
oscillations out of galena, either in a normal cat's whisker detector
configuration or with two point contacts, like the earliest
transistors. Perhaps I have a suitable sample of the mineral to achieve this!
