Lets assume that a voltage U over the ends of the

filament of an incandescent lamp will cause a

current I to flow through the filament. When the

voltage U is increased to U', the current I will

increase from I to I'. The quotient U/I is called the

resistance R of the material and a straight

resistance is independent of external properties like

current, voltage, pressure or temperature. In that

case U/I equals U'/I' and the deduced relation is

known as Ohm's law: "The current through a

conductor between two points is directly

proportional to the potential difference across these

two points" or R=U/I. The resistance R is a

proportional number with unit volt per ampere (V/A).

Instead of V/A commonly the unit Ohm with symbol

Ω is used and the resistance of a conductive

wire with a given cross section can then be

expressed in Ohms per meter (Ω/m). In reality

the resistance of the filament of an incandescent lamp is not linear since it depends

strongly on the temperature of the filament. The temperature coefficient of the

filament of an incandescent lamp is positive, which means that with increasing

temperatures the resistance of the filament will increase too.

When an incandescent heat lamp, like the Vienna Astralux Tiefenstrahler on display

here, was switched on, the temperature of the filament was still relatively low and the

current as a result would be high (this is why incandescent lamps mostly failed at the

moment they were switched on). The supplied energy would cause the temperature of

the filament to rise almost instantly, which increased the resistance and reduces the

initial current. After a short while the circuit stabilised and current and temperature

would remain constant.