I lit a small bulb using two separate power supplies. The yellow battery pack provided a d.c. supply and the other source was low voltage a.c.
I connected both sources to the oscilloscope. The flat line is the d.c. and the wave is the a.c. They both provide the same energy because they lit the bulb by the same amount. So the flat line must show the average voltage of the wave. This type of average is called a root mean square. Since the true mean of such a wave would be zero because it is as much negative as it is positive, we have to square it before we average it to get rid of the negatives. Then the root it back to normal size.
In fact, the flat line is lower than it should be so the two sources can't quite have lit the bulb equally.
