To test a device, connect a resistor to its terminals. If the resistor
gets warm, you have a negative resistance.

The neon lamp and tunnel diode exhibit a negative *dynamic* resistance.
That it, the *slope* of the V/I curve is negative at some parts of the
range. They don't exhibit a true negative resistance -- that is, under
no conditions will you find the value of V/I to be negative.

Roy Lewallen, W7EL

Ralph Mowery wrote:

I was thinking the neon lamp could display negative resistance at certain
voltage/current ranges. The tunnel diode can also.
Not sure right off it they are active devices or not without looking at some
old information I have.