A pendulum swings to and fro. On the left side is
the
Negative phase with the Positive phase on the right. When swinging to
the right the action is expanding to the Positive. So the motion is a
Positive-Positive one. After reaching the full extent of its swing it
begins to collapse back to the Neutral line. This is a negative motion
(to the center) so it is a Positive-Negative phase. Just the opposite
occurs on the left and Negative phase side. The conventional two phases
of a vibration are in actuality four phases. See
fig. 4 where a sine wave is
illustrated showing this dramatic difference and understanding.