Negative resistance

Negative resistance is when a device or material allows more current to flow as voltage across it is *decreased*, or vice versa, less current is passed with an *increase* in voltage. It's principle use to to create very high speed oscillators, in the microwave range.

Some natural semiconductors also seem to have this capability, Cornish arsenopyrite and wolframite seem to occasionally. In a curve plot it looks like a reverse loop, it's hard to capture as it's rare.

This is potentially some negative resistance on a sample of arsenopyrite from Wheal Fortune, Gwennap.

[Edit] From more recent tests, the trace above is more likely to be caused by capacitance - I'm not sure if the test signal used was a sawtooth or a sine (which would reduce this) but more testing is needed to be sure.