See http://en.wikipedia.org/wiki/Mains_electricity and http://www.tpi-thevalueleader.com/rms.html

AC voltage goes from 0 to its peak (120 or 240) and back to 0 (in US, 60 times per second). Graph it with voltage and time and it looks like a camel's hump (sine wave).

So what's the real voltage? Not 120, since it's only there an instant. Not 0, or there would be no voltage. It's somewhere in between since it is always changing. See the rms voltage article.

Your meter can't keep up with changes, so it reads a happy number somewhere in between also.

Many inverters (DC to AC converters) and many types of power supplies, simply chop off the top of the sine and their electronics square off the sides as they switch on and off to give the correct frequency. This is called a square wave.

Other applications may have a saw tooth shape.

They both have enough of what's underneath the camel's hump to work.

Your homework assignment for tomorrow is to tell the group what I just said in a way that makes sense.



Looking at the camel's hump, you can see it isn't a simple average since there's less area under the peak than points under the rest of the graph.