Definition
The effective value of an alternating current (AC) is the value of AC that produces the same heating effect in a resistance as an equal value of direct current (DC). It is equal to 0.707 times the peak (maximum) value of a sine-wave AC, and is also called the root-mean-square (RMS) value.
Plain English
Alternating current is constantly changing — rising, falling, and reversing direction. The effective value is the steady number we use to describe how much real work that changing current does. It is the AC value that would heat a wire the same amount as a steady DC current of the same number.
Context Anchor
Seen in aircraft electrical-system discussions, especially when reading AC voltage or current values from an inverter, alternator, generator, or meter.
Derivation
Called 'effective' because it describes the effect the AC actually produces — specifically, the heating effect — rather than its constantly changing instantaneous value. The alternative name 'root-mean-square' describes the math used to calculate it: square the values, take the average (mean), then take the square root.
Why Pilots Care
Correct meter interpretation prevents misdiagnosis of electrical problems that could affect avionics, lights, or ignition systems.
Grounding Statement
If an AC meter shows 115 volts, it is normally showing the effective value: the AC has the same useful effect as 115 volts of steady DC in the same load.
Intuition Check
Effective does not mean “working well” here. It means the equivalent steady value based on the effect the changing AC produces.
Example Sentence 1
The 115 volts listed for the aircraft's AC bus is the effective value, so the actual peak voltage in the circuit is closer to 163 volts.
Example Sentence 2
A true-RMS meter is needed to read the effective value accurately on the AC portion of the electrical system.