Lighting Systems 335
lights can also be used for outdoor lighting.
The sodium-vapor light, shown in Figure 13-7, is popular for outdoor
lighting and for highway lighting. This lamp contains some low-pressure
neon gas and some sodium. When an electric current is passed through
the heater, electrons are given off. The ionizing circuit causes a positive
charge to be placed on the electrodes. As electrons pass from the heater to
the positive electrodes, the neon gas is ionized. The ionization of the neon
gas produces enough heat to cause the sodium to ionize. A yellowish light
is produced by the sodium vapor. The sodium-vapor light can produce
about three times the candlepower per watt as an incandescent light.Figure 13-6. A mercury-vapor lampFigure 13-7. A sodium-vapor lamp.