Electrical ballast

An American electronic instant start ballast for powering a variety of American T8 fluorescent lamps.
Variety of ballasts for fluorescent and other discharge lamps

An electrical ballast is a device placed in series with a load to limit the amount of current in an electrical circuit.

A familiar and widely used example is the inductive ballast used in fluorescent lamps to limit the current through the tube, which would otherwise rise to a destructive level due to the negative differential resistance of the tube's voltage-current characteristic.

Ballasts vary greatly in complexity. They may be as simple as a resistor, inductor, or capacitor (or a combination of these) wired in series with the lamp; or as complex as the electronic ballasts used in compact fluorescent lamps (CFLs).