
Background
Ohm discovered that the voltage (V) across a resistor changes as the current
(I) passes through the resistor. The relationship between V and I can
be expressed as
where R is the resistance
of the resistor. In other words, as the voltage increases, so does the
current. The proportionality constant is the value of the resistance.


Nonohmic material
For a light bulb, the resistance of the filament will change as it heats
up and cools down. At high a.c. frequencies, the filament doesn't have
time to cool down, so it remains at a nearly constant temperature and
the resistance stays relatively constant. At low frequencies, the filament
has time to change temperature. As a result, the resistance of the filament
changes dramatically.
