1. To find voltage: E = IR
2. To find amperage: I = E/ R
3. 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 answer 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.