Dr. Kern Comtesse V52 sunlamp
TEMPERATURE-COEFFICIENT
The filament of an incandescent lamp had a positive
temperature coefficient which means that the
electrical resistance of the filament increased with
temperature. In a burning lamp the resistance of the
filament would increase due to a rise of
temperature, caused by the current through the
filament. According Ohm's law, the current would
then decrease and the temperature of the filament
would decrease subsequently. This effect stabilised
both the current through- and the temperature óf the
filament.
An electrical gas discharge often lacks this
stabilising behaviour. The electrical resistance of a
gas discharge may decrease with increasing
currents and/or temperatures, depending on the
actual size of the current. When the resistance
decreases while the voltage remains the same, the
current increases as a result and the dissipated
energy and hence the temperature of the medium increases. Due to a negative
temperature coefficient and/or a negative voltage-current characteristic, the
resistance may decreases even further, which then leads to a further increase in
current. This process may continue until current and/or temperature reach the point
where the tube or the electrodes will break down. To interrupt this devastating vicious
circle the current had to be limited to a save level and the simplest way to achieve
this was to connect a stabilising electric ballast in series with the discharge tube. The
impedance of such a ballast had to increase with rising currents or temperatures or
at least it had to stay the same. The most commonly used serial ballast elements for
gas discharge tubes were resistors, choke coils or quartz tube heating elements as
with the Dr. Kern Comtesse V52 sunlamp on display here.
