"DJ Delorie" wrote: (clip) My primary motivation, other than to have some
fun on a Friday, is to keep people from confusing single-phase house current
with the original two-phase (4 wire, 90 degrees) AC invented by Tesla. This
2-phase was replaced with our current 3-phase (3 wire, 120 degrees) power.
^^^^^^^^^^^^^^^^
I don't think anyone was trying to say that 220v power coming to the house
is "two phase." Two phase power requires a 90 degree lag (or lead) between
the phases.

If you look at two voltages on an oscilloscope, and see one reaching its
positive peak while the other is reaching its negative peak, is one 180
degrees out of phase with the other? According to your thinking, it
depends on the source. If its coming from a center-tapped transformer, it's
not a phase difference, but a polarity reversal. If it's coming from
generator windings that are positioned on opposite sides of the stator, it
would be a 180 degree phase difference.

Suppose its coming out of a black box?