Open: An open circuit is one that is not complete and lacks a return path to the source-a switch opens and closes a circuit and shuts off the load. A broken wire or a burned out bulb also opens a circuit.

Short: A short is a complete circuit that goes to ground before the load. The danger in a short is this-because it goes to ground before the load, or the resistive device that consumes electricity, the resistance in the circuit is often drastically reduced; as a result current flow increases which can create enough heat to melt the wire or in the worst case scenario, start a fire.

Series: This is where electricity flows through one load then another. As an example, if light bulbs are were wired in series and one bulb burned out, both would go out.

Parallel: Headlights, as an example, are wired in parallel. Wires from the switch split and go to each headlight separately. If one headlight goes out the other is not affected.

Electrical Devices
There are all sorts of electrical devices found in trucks, the common ones are:

Fuses: These are protective devices that open the circuit when current flow exceeds a specified amount. Fuses are rated by the circuit's current-carrying capacity. One of the most dangerous things that can be done is to increase the amp rating of a fuse to keep it from "blowing." The result can be more current flowing through the wire than it is capable of carrying that can cause the wire to burn before the fuse opens the circuit.

Relays: A relay is an electrically controlled switch that lets a light-duty circuit control a heavier circuit. Used on loads that draw lots of current, such as an engine-cooling fan, a light-duty switch with relatively light wire can turn the relay on and off. The relay uses heavier wire from the battery to the load, and it turns the fan on and off.

Solid-State Devices
The term solid-state means that an electrical device has no moving parts other than electrons. Some of the common solid-state devices found in trucks are:

Diodes: These are one-way electrical valves, current can flow one way but not the other. Diodes are found in a number of places, most notably in alternators; they help transform alternating current to direct current.

Light Emitting Diodes: Becoming more common all the time, LEDs have a small lens built in. The flow of electrons generates energy released in the form of light. Unlike a conventional bulb, no heat is created.

Transistors: These are solid-state switches used in applications like electronic ignitions to turn a circuit on and off, or in a stereo to strengthen a radio signal.
Resistors: Used to limit current flow, resistors can have a fixed value or they can be adjustable (also called a potentiometer).

Capacitors: A capacitor can store an electric charge. The condenser in a point style ignition is actually a capacitor. Some electronic ignition systems use a capacitor to store an electrical charge that is released all at once.

Making Electricity
There's an interesting relationship between electricity and magnetism-When electricity is flowing through a wire, there is a small magnetic field generated around the wire, and when a wire moves through a magnetic field, a small amount of current or electricity is generated in the wire. Creating electricity is simple a matter of moving one past another continuously. A generator does it by moving a bunch of wire loops, called an armature, inside a fixed magnetic field. The amount of electricity produced is controlled by the rpm of the armature and the strength of the magnetic field. An alternator spins a series of windings, called a stator, inside a magnetic field. Alternators produce AC current, but an internal device called a rectifier converts it to DC.