How do you measure effective value of an alternating voltage?

The "effective" value of an alternating voltage is generally considered to be the RMS (Root-Mean-Square) value. The best way to measure that is with a True RMS voltmeter.

Lacking that, if the voltage is sinusoidal, you can use an older style peak measuring voltmeter that estimates RMS value by dividing internally by the square root of 2. Any other shaped waveform will be measured incorrectly, depending on the amount of deviation from sinusoidal. (Square wave is the best example of error in this case

• RMS and peak should be the same, but they won't read the same

except on a True RMS voltmeter.)