Several people have given good examples of non-ohmic devices: diodes,
transistors, capacitors, gas discharge tubes...
Also mentioned was light bulbs, and separately mentioned: the
temperature dependence of resistance. Actually, I think tungsten is
generally regarded as ohmic. The fact that the resistivity is
temperature dependent is not sufficient to claim something is
non-ohmic. If it were, there would be essentially no ohmic materials.
I think ohmic means the material has a current that is a linear
function of the applied potential difference assuming temperature
remains constant. To be non-ohmic requires that the current be a
non-linear function of the potential difference because of things that
happen due to voltage or due to current density, not because of other
things that happen such as temperature changes.
In a lab that my students do, they find that a 1/4-watt carbon-film
resistor displays some non-ohmic tendency if they dissipate 1-2 watts
in it (nearly smoke it). But data over the same range of potential
differences become remarkably ohmic if they put a heatsink on the
resistor. However, a zener diode cannot be made to appear ohmic with
addition of a heatsink nor any manner of waving of hands or uttering of
incantations.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817