334 Electrical Power Systems Technology
heat produced by current flow through the heater. The filament circuit is
now opened. A capacitor is connected across the bimetallic switch to re-
duce contact sparking. Once the contacts of the starter open, a high volt-
age momentarily occurs between the filaments of the lamp, because of
the action of the inductive ballast coil. The ballast coil has many turns of
small-diameter wire, and, thus, it produces a high, counter-electromotive
force when the contacts of the starter separate. This effect is sometimes
called “inductive kickback.” The high voltage across the filaments causes
the mercury to ionize and initiates a flow of current through the tube.
There are several other methods used to start fluorescent lights; however,
this method illustrates the basic operating principle.
Fluor escent lights produce more light per watt than incandescent
lights; therefore, they are cheaper to operate. Since the illumination is pro-
duced by a long tube, there is also less glare. The light produced by fluo-
rescent bulbs is very similar to natural daylight. The light is whiter, and
the operating temperature is much lower with fluorescent than with in-
candescent lights. Various sizes and shapes of fluorescent lights are avail-
able. The bulb sizes are expressed in eighths of an inch, with the common
sizes being T-12 and T-8. (A T-12 bulb is 1-1/2 inches.) Common lengths
are 24, 48, 72, and 96 inches.
Vapor Lighting
Another popular form of lighting is the vapor type. The mercury-va-
por light is one of the most common types of vapor light. Another com-
mon type is the sodium-vapor light. These lights are filled with a gas that
produces a characteristic color. For instance, mercury vapor produces a
greenish-blue light, and argon a bluish white light. Gases may be mixed to
produce various color combinations for vapor lighting. This is often done
with signs used for advertising.
A mercury-vapor lamp is shown in Figure 13-6. It consists of two
tubes with an arc tube placed inside an outer bulb. The inner tube con-
tains mercury. When a voltage is applied between the starting probe and
an electrode, an arc is started between them. The arc current is limited
by a series resistor; however, the current is sufficient to cause the mer-
cury in the inner tube to ionize. Once the mercury has ionized, an intense
greenish-blue light is produced. Mercury-vapor lights are compact, long-
lasting, and easy to maintain. They are used to provide a high-intensity
light output. At low voltages, mercury is slow to vaporize, so these lamps
require a long starting time (sometimes 4 to 8 minutes). Mercury-vapor