Sparky,
I am going to give an extremely brief answer here first, then type up a more in depth message off-line in my word processor, which I'll post later. This might explain the stuff behind your question better.
Just briefly, you are measuring the amplitude of one flow direction - either the one above the zero line, or the one below it. The meter will most certainly show the RMS [Root Means Square] value of that 1/2 cycle. The RMS value of a half cycle is .707 of the peak value. RMS is the AC equivalent to DC when heating is concerned [this is way too brief!! I am truly sorry!].
If the Peak value was 170 volts, the RMS value would be 120 volts [and vice verse]. There's another value, called Average Value, which is something like .63 of the Peak value [I haven't used that value for so long, I can't remember it's percentage! I'll check on it and include with next message].
Very rarely will one need to use Peak to Peak values. This would be something of concern for Rectifiers or maybe Capacitors.
I'll post a better message later tonight!!
Bill,
Great idea having an area devoted to theory! I am sure the rest agree!
Scott.
