1. Ohm's law says that the current through a device is directly
proportional to the applied potential difference.
2. Ohm's law is not true in general.
3. Devices that *do* obey Ohm's law are called "ohmic" or
"linear."
4. The resistance of a device is defined by R = delta_V/I
5. Therefore, Ohmic devices have a resistance that is independent
of delta_V or I. (You could, for instance, label them permanently
with a value that represents the ratio of delta_V to I or simply
band them with stripes that code that value.)