The lighting circuit (fig. 2-54) includes the battery, vehicle frame, all the lights, and various switches that control their use. The lighting circuit is known as a single-wire system since it uses the vehicle frame for the return.

The complete lighting circuit of a vehicle can be broken down into individual circuits, each having one or more lights and switches. In each separate circuit, the lights are connected in parallel, and the controlling switch is in series between the group of lights and the battery.

The marker lights, for example, are connected in parallel and are controlled by a single switch. In some installations, one switch controls the connections to the battery, while a selector switch determines which of two circuits is energized. The headlights, with their high and low beams, are an example of this type of circuit.

In some instances, such as the courtesy lights, several switches may be connected in parallel so that any switch may be used to turn on the light.

When a wiring diagram is being studied, all light circuits can be traced from the battery through the ammeter to the switch (or switches) to the individual light.

Figure 2-55.- Lamp construction and configurations.

LAMPS

Small gas-filled incandescent lamps with tungsten filaments are used on automotive and construction equipment (fig. 2-55). The filaments supply the light when sufficient current is flowing through them. They are designed to operate on a low voltage current of 12 or 24 volts, depending upon the voltage of the the vehicle will be of the single-or double-contact small one-half-candlepower bulbs to large 50- candlepower bulbs. The greater the candlepower of the lamp, the more current it requires when lighted. Lamps are identified by a number on the base. When you replace a lamp in a vehicle, be sure the new lamp is of the proper rating. The lamps within Lamps are rated as to size by the candlepower (luminous intensity) they produce. They range from types with nibs to fit bayonet sockets, as shown in lamp is also whiter than a conventional lamp, which increases lighting ability.

Figure 2-56.-( A) Single-contact bulb; (B) Double-contact bulb.

Figure 2-58.- Dash-mounted headlight switch.