6.3 Electric Measurements

Electricity is measured in two ways—by the amount of current (number of electrons) flowing and by the push, or pressure, that causes current to flow. The push, or pressure, is caused by actions of the electrons. They repel each other. When electrons are concentrated in one place, their negative charges push against each other. If a path is provided for the electrons, they will flow away from the area where they are concentrated.

The pressure to make them move is called voltage. If there are many electrons concentrated in one spot, we say that there is high voltage. With high voltage, many electrons will flow, provided there is a path or conductor through which they can flow- the more electrons that flow, the greater the electric current. Electric current is measured in amperes. Resistance is the movement of electrons through a substance.

Resistance is a fact of life in electric circuits. We want resistance in some circuits so that too much current (too many electrons) will not flow. In other circuits, we want as little resistance as possible so that high current can flow.

There is a definite relation between current (electron flow), voltage (current pressure), and resistance. As the electric pressure goes up, more electrons flow. Increasing the voltage increases the amperes of current. However, increasing the resistance decreases the amount of current that flows. These relationships can be summed up in a statement known as Ohm's law.

Ohm’s Law

Ohm’s law is used to figure out the current (I), the voltage (E), and the resistance (R) in a circuit. This law states that voltage is equal to amperage times ohms. Or, it can be stated as the mathematical formula: E = I x R. For the purpose of solving problems, the Ohm’s law formula can be expressed in three ways:

• To find voltage: E = IR

• To find amperage: I = E/R

• To find ohms: R = E/I

The Ohm’s law formula is a useful one to remember because it helps in understanding the many things that occur in an electric circuit. For example, if the voltage remains constant, the current flow goes down if the resistance goes up. This can be better explained by using a truck lighting circuit that is going bad. Suppose the wiring circuit between the battery and the lights has deteriorated due to connections becoming poor, strands in the wire breaking, and switch contacts becoming dirty. All of these conditions reduce the electron path or, in other words, increase resistance. This increased resistance decreases the current flow with the battery voltage constant (for example, 12 volts). If the resistance of the circuit when new was 6 ohms, then 2 amperes will flow. To solve the equation, 12 (volts) must equal 12 (amperes times ohms). But if the resistance goes up to 8 ohms, only 1.5 amperes can flow. The increased resistance cuts down the current flow and, consequently, the amount of light.

If the resistance stays the same but the voltage increases, the amperage also increases. This is a condition that might occur if a generator voltage regulator became defective. In such a case, there would be nothing to hold the generator voltage within limits, and the voltage might increase excessively. This would force excessive amounts of current through various circuits and cause serious damage. If too much current went through the light bulb filaments, for example, the filaments would overheat and burn out. Also, other electrical devices probably would be damaged. However, if the voltage is reduced, the amount of current flowing in a circuit will also be reduced if the resistance stays the same.

For example, with a run-down battery, battery voltage will drop excessively with a heavy discharge. When you are trying to start an engine with a run-down battery, the voltage will drop very low. This voltage is so low that it cannot push enough current through the starter for effective starting of the engine.


Electrons are caused to flow by a difference in electron balance in a circuit, that is, when there are more electrons in one part of a circuit than in the other, the electrons move from the area where they are concentrated to the area they are lacking. This difference in electron concentration is called potential difference, or voltage. The higher

the voltage goes, the greater the electron imbalance becomes. The greater this electron imbalance, the harder the push on the electrons (more electrons repelling each other) and the greater the current of electrons in the circuit. When there are many electrons concentrated at the negative terminal of a generator (with a corresponding lack of electrons at the positive terminal), there is a much stronger repelling force on the electrons; consequently, many more electrons are moving in the wire. This is exactly the same as saying that the higher the voltage, the more the electric current will flow in a circuit, all other things, such as resistance, being equal.


Current flow, or electron flow, is measured in amperes. While we normally consider that one ampere is a rather small current of electricity (approximately what a 100-watt light bulb would draw), it is actually a tremendous flow of electrons. More than 6 billion electrons a second are required to make up one ampere.


A copper wire conducts electricity with relative ease; however, it offers resistance to electron flow. This resistance is caused by the energy required to free the outer shell of electrons and the collision between the atoms of the conductor and the free electrons. It takes electromotive force (emf) or voltage to overcome the resistance met by the flowing electrons. The basic unit of resistance is the ohm. The resistance of a conductor varies with its length, diameter, composition, and temperature. A long wire offers more resistance than a short wire of the same diameter; this is due to the electrons having farther to travel. Some materials can lose electrons more readily than others. Copper loses electrons easily, so there are always many free electrons in a copper wire. Other materials, such as iron, do not lose their electrons as easily, so there are fewer free electrons in an iron wire. However, fewer electrons can push through an iron wire, that is, the iron wire has more resistance than the copper wire. A wire with a small diameter

offers more resistance than a wire with a large diameter. In the small diameter wire, there are fewer free electrons, and thus fewer electrons can push through. Most metals show an increase in resistance with an increase in temperature, while most nonmetals show a decrease in resistance with an increase in temperature.

Circuit Configurations

Automotive Circuits

The body and chassis of an automobile are made of steel. This feature is used to eliminate one of the wires from all of the automobile circuits. By attaching one of the battery terminals to the body and chassis, you can connect any electrical component by hooking up one side, by wire, to the car battery and the other side to the body (Figure 6-13).

Figure 6-13 — Automotive circuits.

This design of connecting one side of the battery to the automobile body is called grounding. The majority of equipment you will encounter has an electrical system with a negative ground. Vehicles with positive ground are very uncommon, but it is always good practice to note what type of grounding system is used on the equipment you are working on.

Series Circuits

A series circuit consists of two or more electrical components connected in such a manner that current will flow through all the components. Important characteristics of a series circuit are as follows:

• Any break in the circuit (such as a burned-out light bulb) will render’ the entire circuit inoperative.

• Current (amperage) will be constant throughout the circuit.

• Total resistance of the circuit is equal to the sum of each individual resistance.

• Total voltage of the circuit is equal to the sum of the individual voltage drops across each component.

Parallel Circuits

A parallel circuit consists of two or more electrically operated components connected by parallel wires (Figure 6-14). In a parallel circuit, the current divides, part of it flowing into one component and part into the others. The same voltage is applied to each component, and each component can be turned on or off independently of the others. Important characteristics of parallel circuits are as follows:

• The total resistance of the circuit will always be less than the resistance of any individual component.

• The disconnection or burning out of any individual component in the circuit will not affect the operation of the others.

Figure 6-14 — Parallel circuit.

• The current will divide itself among the circuit branches according to the resistance of the individual devices. The sum of the individual amperages will be equal to the total circuit current.

• The voltage will be constant throughout the circuit when measured across the individual branches.

Series-parallel Circuits

The series-parallel circuit is a combination of series circuits and parallel circuits (Figure 6-15). There must be at least three resistance units to have a series-parallel circuit.

Figure 6-15 — Series-parallel circuit.

Important characteristics of series-parallel circuits are as follows:

• The total circuit voltage will be equal to the sum of the total parallel circuit voltage drop plus the voltage drop of the individual series circuit component.

• The total circuit resistance will be equal to the sum of the total parallel circuit resistance plus the individual resistance of the series circuit components.

• Current flow through the total parallel circuit will be equal to the current flow through any individual series circuit component.

• The disconnection or the burning out of the series components will completely disable the entire circuit, whereas a failure of any of the parallel circuit components will leave the balance of the circuit still functioning.

Circuit Failures

Open Circuit

An open circuit is a break or interruption in the circuit, such as a wire that has come loose or a slipped connection that is not making contact. But the expression of an open circuit is not only used when wire connections are actually separated as in a switch but also when the resistance in the wiring circuit is such that no current can flow between the battery and the unit it operates. Some good examples of such a condition are rust and corrosion that form and accumulate at a battery cable or terminal, or a fuse failure.

Short Circuit

A short circuit occurs when copper touches copper, such as when wiring insulation between two wires fails and the wiring makes contact. These are undesirable and lead to overheating an electrical circuit. The excessive current flow caused by short circuits overheats the wiring harness and can cause vehicle fire.

Ground Circuit

A ground circuit occurs when any part of the wiring circuit is touching the vehicle frame inadvertently. A ground involves accidental or unintentional contact between copper and the iron frame. This too can lead to excessive current flow and overheating.

Test your Knowledge

4. To have a series-parallel circuit, you must have what minimum number of resistance units?

A. One
B. Two
C. Three
D. Four

5. What type of circuit failure occurs when the resistance in the wiring circuit is such that current CANNOT flow between the battery and the unit it operates?

A. Short circuit
B. Open circuit
C. Ground circuit
D. Dead circuit