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Basic Electricity

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Electricity is the flow of electrons through a circuit. 
The electrons, which flow through a buses wiring circuits are carried by conductors.  Wires lead electrons to, and through, loads.
A load is something that provides resistance to the flow of electrons, in so doing, makes the electrons do useful work as they travel.
A complete circuit is necessary if electricity is to flow.
A circuit includes a Power Source (battery), a Load (a.c. blower), a Conductor (wiring), and a Ground (negative battery post, connected to the bus frame).
The wire carries current from the positive side of the battery, to the load, under electrical pressure, better known as Voltage.
The ground wire then carries the current back to the negative side of the battery.
All four parts of the circuit must be present for electricity to exist.
The actual flow of electricity is from negative to positive.
THe electrons have a negative charge, thus they cling to the negative side of the battery and are drawn throught the circuit because the positive side is pulling them through.
But it is easier to work backwards for troubleshooting purposes.
Control Devices
To control the flow of electrons, switches are placed through out the circuit.
A switch stops, or redirects, the flow of electons by opening or closing it's contacts.  
Another control device is a Fuse.  It is for safety.
A fuse is made up of a piece of metal that has a low melting point.
When the electron flow becomes too great for the wiring to handle, resistance increases and that creates heat.  Soon, very very fast in fact, the metal in the fuse gets hot and melts, opening the circuit and eliminating the current flow.
This prevents fires.
Another control Device is a circuit breaker.
A CB does the same thing as a fuse, but is reusable.
Instead of melting, it has contacts inside that open to interupt the circuit. Typically, the contacts are opened by a device that responds to heat, a result of current flow.
A major variable that affects resistance in a circuit is wire diameter.  A large diameter wire allows more electrons too flow and can handle more heat.  For example, an AC blower motor requires more current to operate than a headlight. 
So it's wiring is much larger than a headlight.  If you put a small headliht wire on a Starter Motor, it will melt because it will resist the current flow and generate heat.
Voltage would also drop through the wire.
When properly sized, carry current (amps) throught the circuit with very little resistance.  A hot electrical wire means that more electrons are trying to flow through than the wire is capable of carrying.  Like too many cars on the highway.
Voltage is the electrical pressure that pushes the flow of electrons through a circuit. Also known as the Potential.  Voltage is not electricity, it is the Potential for electricity.
Amps is the unit of measure for current flow.  You can have voltage all day long, but with no amps, you have no electricity.  Too often, we mechanics overlook testing a circuits amps.  We see voltage and assume the circuit is good. 
Remember to check amps, your meter has to be connected to the circuit in Series, which means it literally has to become part of the circuit.  The easy way to do that is to test at a fuse.  Remove the fuse and put your multimeter leads in where the fuse was.  Your meter is now part of the circuit.
Reistance is measured in Ohms.  It is what holds back the flow of electrons and makes the current do useful work for us.  It is good and bad.  Resistance is bad in the form of corrosion, frayed or partially cut wires, defective motors.
additional resistance in a circuit causes Voltage Drop.  That means there will be less voltage to push the amps through.  It is caused by poor connections, corrosion, paint between a connector and ground, any poor connection.
A drop of more than one volt through any one section of a circuit indicates a problem. 
Continuity is just a complete circuit.  If you check a circuit for Continuity, you are checking to make sure it is all connected.  Continuity checks are simple, and therefore everyones favorite.  However, continuity checks do not tell you if a circuit is carrying proper voltage or amps. 
A short is when a wire rubs or in anyway makes contact with another circuit, therefore recieving it's power too soon.
Short to ground is when a circuit rubs ,or makes contact with ground before it reaches it's load, rendering it ineffective.

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