Electric Wiring, 3rd Edition

Fluorescent-tube lighting 53

of evaporation of barium from the cathode and also to reduce the starting

voltage.

The tube cathodes are heated by passing a current through each filament,

which causes

a

large number of

electrons

to

be

emitted

by

the oxide coatings.

Assuming sufficient voltage is applied across the tube,

a

glow discharge

is

set

up through the argon gas which excites or ionises the mercury atoms

throughout the tube and establishes a mercury-arc discharge.

All fluorescent tubes require appropriate control gear for operation: a

ballast to limit current through the tube, a starting device for preheating the

cathode filament and, usually ...