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Vienna Astralux heat lamp
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.