
3.2 Low-Pressure Discharge Lamps 59
The dimensions of the electrode and more specically of the tungsten lament
depend on the application of the lamp and its power.
3.2.1.3 The Ballast
An operating gas discharge is a nonlinear circuit element that cannot be described
by Ohm’s law (V=IR) because the conductivity increases with current. The dis-
charge is therefore said to exhibit negative differential voltage-current charac-
teristics, except in some special cases such as weakly ionized plasma. A current
limiting device, called a ballast, is therefore necessary for the operation of dis-
charge lamps. In a discharge tube, the low-pressure gas ...