Resistance

This lecture is based on HRW, Sections 27.4-6

Materials in which the current is linearly proportional to the electric flux are called ohmic. Another way to say it is that the resistance of an ohmic material doesn't depend upon the applied voltage.
For such materials, the factor relating voltage to current may be expressed either as conductivity, or resistivity. This changes by small amounts with temperature.
Given a wire of specified length and cross-section, one can define resistance as the ratio of current to voltage within that wire.
Some materials lose all resistance to the flow of current when their temperature drops below a critical value; we call those superconductors.

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