Electrical Power Systems Technology

Lighting Systems 333

in Figure 13-4. The tube is filled with mercury vapor; thus, when an elec-

trical current flows through the two filaments, a continuous arc is formed

between them by the mercury vapor. High-velocity electrons passing be-

tween the filaments collide with the mercury atoms, producing an ultra-

violet radiation. The inside of the tube has a phosphor coating that reacts

with the ultraviolet radiation to produce visible light.

Figure 13-4. Operating principle of a fluorescent lamp

One circuit for a fluorescent light is shown in Figure 13-5. Note that

a thermal starter, which is basically a bimetallic strip and a heater, is con-

nected in series with the filaments. The bimetallic strip remains closed

long enough for the filaments to heat and vaporize the mercury in the

tube. The bimetallic switch will then bend and open as the result of the

Figure 13-5. Circuit diagram of a fluorescent lamp